Middle English bitwixe, from Old English betweox "between, in the space that separates, among, amidst, meanwhile," from bi- "by" (see by) + tweox "for two," from Proto-Germanic *twa "two" (from PIE root *dwo- "two") + *-isk "-ish."
With unetymological -t that appeared in Old English and became general after c. 1500. Compare amidst. Betwixen also was a variant in Old and Middle English. Middle English also had twix (prep., adv.) "among; in the meantime." Now mostly in the colloquial intensive expression betwixt and between.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dvau, Avestan dva, Greek duo, Latin duo, Old Welsh dou, Lithuanian dvi, Old Church Slavonic duva, Old English twa, twegen, German zwei, Gothic twai "two;" first element in Hittite ta-ugash "two years old."