Sp. dos and Eng. two
Spanish dos /ˈd̪os/ is a patrimonial descendant of Latin dŭōs ‘two’, which is the accusative, singular, masculine form of this numeral, whose nominative masculine form was dŭō. The change from Lat. dŭōs to Sp. dos is just what we would expect in a patrimonial word, for Latin short ŭ always became o in Spanish and long vowels reduced to a single one: doos > dos. Final ‑s was not lost, unlike final ‑m, which is the more common final consonant found in accusative singular words.
Lat. dŭō descends from the word for ‘two’ in Proto-Indo-European was *dwóh₁. Other descendants of this word include Ancient Greek δύο (dúo) and Proto-Germanic *twai, which is the source of Old English twā, which is the source of Modern English two. Here we can see the ubiquitous sound change from Proto-Indo-European d to Proto-Germanic t. Eng. two is now pronounced [ˈtu], but it used to be pronounced [ˈtwo], as its spelling indicates.
Thus, Sp. dos and Eng. two are patrimonial cognates, that is, patrimonial (not borrowed) words in each language that descend from the same source, in this case Proto-Indo-European, the ancestor language of all Indo-European languages (cf. Part I, Chapter 3). Since numbers are hardly ever replaced in language, other Indo-European languages also have cognates for this numeral, such as French deux, pronounced [ˈdø(z)] (from Old French deus), Russian два (dva), Sanskrit द्व (dvá), German zwei or zwo, Danish and Norwegian to, Swedish två or tu, etc.
Old Spanish had a feminine form for the numeral dos, which is attested in early writings as either dues or duas. It came from the feminine accusative form dŭās of the Latin numeral (nominative: duae). Obviously, this wordform has not survived into Modern Spanish.
Lat. dŭō has been borrowed by English and Spanish in recent times as Eng. duo (pronounced [ˈdu.oʊ̯] or [ˈdju.oʊ̯]) ~ Sp. dúo ([ˈdu.o]) to refer to ‘a pair of people or things, especially in music or entertainment’ (COED). English also has a word duet, which is a mid-18th century loanword from French duet, which itself is a loanword from Italian duetto, which is a diminutive of Italian duo ‘two’, a direct descendant of Lat. dŭō. The word duet has the same meaning as one of the meanings of duo, namely ‘a performance by two singers, instrumentalists, or dancers’ and, from there, ‘a musical composition for two performers’ (COED). Spanish has also borrowed the word dueto from the Italian, as a partial synonym of dúo, since it means ‘music duo or song sung by two voices’, though the word dúo is more commonly used.
Lat. bi-, Gk. di-, and Germanic twi-
Latin had an adverb bis that meant ‘twice’ (‘two times’). This adverb was derived from an earlier dŭis (dŭ‑is), related to the numeral dŭō ‘two’ (dŭ‑ō). The sound change from dŭ to b word-initially before a vowel is not uncommon in Old Latin. Thus, for instance, Lat. bellum ‘war’ derives from an earlier duellum and Lat. bonus ‘good’ derives from an earlier duonus, itself coming from an earlier duenus. This word duonus, was fully replaced by bonus, but curiously, duellum was retained alongside bellum in writing and it was brought back into Medieval Latin to refer to a combat between two people. From there it was borrowed by both English and Spanish. Learned Spanish duelo is first attested in the late 15th century and English borrowed the word duel in the late 16th century. (There is a second, unrelated patrimonial word duelo in Spanish that means ‘sorrow, grief’ and ‘mourning’. This word is related to the verb doler ‘to hurt’, ‘to be sorry, be sad’, ‘to distress, sadden’, cf. the expression estar de duelo ‘to be in mourning’.)
The Latin adverb bis became a prefix bi‑ in Latin, which meant ‘having two parts’ or ‘occurring twice’. We can still recognize this prefix in many derived Spanish and English words, mostly New Latin words, such as Eng. bilingual ~ Sp. bilingüe, and scientific words for chemical compounds, such as Eng. bicarbonate ~ Sp. bicarbonato. (Sp. bicarbonato is the popular word for baking soda, which chemically is sodium bicarbonate.)
A classical Latin word containing this prefix is biennium (bi+ann(us)+ium) ‘two years’, which has given us the learned Sp. bienio ~ Eng. biennium ‘two-year period’. More common than Eng. biennium is the derived adjective Eng. biennial ‘taking place every other year’ or, in botany, ‘having a two year cycle’ or ‘lasting two years’. Spanish also has a cognate adjective bienal, which can also be used as a (feminine) noun meaning ‘biennial exhibition’ (‘exposición o manifestación artística o cultural que se repite cada dos años’, DLE). There are also the adjectives Eng. biannual ~ Sp. bianual, which primarily mean ‘occurring twice a year’ (synonymous with Eng. semiannual ~ semianual), but which in bother languages can also be used as synonyms of biennial/bienal ‘occurring every other year’. These adjectives are recent, 19th century creations out of the prefix bi‑ and the adjectives Eng. annual ~ Sp. anual.
The prefix bi‑ is used in many new coinages, sometimes with non-Latin roots, as in Eng. bimonthly ‘occurring every other month’ (cf. Sp. bimensual) and biweekly ‘occurring every other week’ (Cf. Sp. quincenal, bisemanal). Other common cognates containing this prefix are: Eng. bilabial ~ Sp. bilabial (in phonetics, ‘articulated with both lips’), Eng. bifurcate ~ Sp. bifurcar(se) (from Lat. bifurcātus ‘forked in two; bifurcated’, from the Latin adjective bifurcus ‘two-forked’), Eng. bicycle ~ Sp. bicicleta (an 1899 loan from Fr. bicyclette, diminutive of bicycle (1868), now archaic in French but which was presumably borrowed into English (OED), though it has also been claimed that the word was first coined in English from which it was borrowed into French), Eng. bipartite ~ Sp. bipartito/a ‘having or consisting of two parts’ (AHD), Eng. bicephalous ~ Sp. bicéfalo ‘two-headed’, Eng. biplane ~ Sp. biplano ‘an early type of aircraft with two pairs of wings, one above the other’ (COED), Eng. bisexual (1824) ~ Sp. bisexual ‘sexually attracted to both men and women’ or, in biology, ‘having characteristics of both sexes’ (COED), Eng. bipolar ~ Sp. bipolar ‘two-poled’, Eng. bifocal ~ Sp. bifocal ‘having two foci’, Eng. bissextile (fancy) ~ Sp. bisiesto ‘leap (year)’ (derived from bis sextus dies ‘the double/twice sixth day [before the calends of March, i.e. February 24]’).
In borrowings from Latin and patrimonial Spanish words sometimes we find the alternate versions (allomorphs) of this suffix bis‑ and biz‑: as in bisabuelo ‘great-grandfather’ and biznieto ‘great-grandson’. The version bis‑ is found in English occasionally before c, s, or a vowel, or in the names of chemical compounds, though this variant is mostly obsolete.
The Latin adverb bis goes back to Proto-Indo-European *duis ‘twice’. Thus, it is not surprising that it has cognates in other Indo-European languages and, in particular, in Greek and Germanic. In all of them the prefix contains the vowel i. The Latin prefix bi‑ is cognate with the Greek prefix di‑, from Ancient Greek δίς (dís) ‘twice’, which is a cognate of Latin bis. This prefix is found in Greek borrowings in English and Spanish, particularly in New Latin creations in scientific vocabulary, such as dichromatic ~ dicromático ‘two-colored’, dioxide ~ dióxido ‘two-oxigens’, and diphthong ~ diptongo ‘two-sound’.
The Germanic family of languages also had an equivalent of this adverb, which has been reconstructed as *twiz ‘twice’ in Proto-Germanic. This is the source of the patrimonial Eng. twice (Sp. dos veces). The adverb twice in English is a cognate of Latin bis and Greek dis. (Remember that Proto-Indo-European d changed to t in Germanic.) This adverb has also been used as a prefix in Germanic languages, meaning either ‘double’ or ‘half’. Curiously, it has only survived in one common English word, namely twilight, ‘the soft glowing light from the sky when the sun is below the horizon, caused by the reflection of the sun's rays by the atmosphere’ (COED). (In twilight, the meaning of twi‑ was probably originally ‘half’, not ‘double’.) The prefix is no longer productive in English, since it has been replaced by the numeral two, forming compounds, but it is found in obsolete or archaic formations such as twi-tongued ‘two-tongued’ or twi-headed ‘two-headed’. The morpheme also surfaces, of course, in the word twin, which comes from Old English twinn, meaning ‘twin, two-fold, double’. It goes back to Proto-Germanic *twinjaz or *twinaz ‘two each’, which has been reconstructed as *dwino‑ in Proto-Indo-European There are a couple of other rare words with this prefix which you will find in the dictionary, such as twi-headed and twi-natured. Thus, we can see that English twi‑ is analogous to Early Latin dui‑ and, thus, Latin bi‑.
 As the OED explains, ‘[t]he Latin form duellum is well attested in ancient inscriptions and retained in classical Latin literature in archaic language and in poetry; in post-classical Latin it is also used specifically to denote a fight between two combatants’.