Old English twinn "consisting of two, twofold, double, two-by-two," from Proto-Germanic *twisnjaz "double" (source also of Old Norse tvinnr "double, twin," Old Danish tvinling, Dutch tweeling, German zwillung), from PIE *dwisno- (source also of Latin bini "two each," Lithuanian dvynu "twins"), from *dwi- "double," from root *dwo- "two." Minneapolis and St. Paul in Minnesota have been the Twin Cities since 1883, but the phrase was used earlier of Rock Island and Davenport (1856).
"to combine two things closely, join, couple," late 14c., from twin (adj.). Related: Twinned; twinning. In Middle English, the verb earlier and typically meant "to part, part with, separate from, estrange," etc. (c. 1200), on the notion of making two what was one.
c. 1300, from Old English getwinn "double;" getwinnas "twins, two born at one birth," from twinn (see twin (adj.)).