It is true that, etymologically, Zwieback and Biscuit are exact correspondants, yet here in Switzerland, they are well-distinct items on the bakery shelves. Zwieback is a sort of bread in slices that is made durable by dehydration, while Biscuits are (generally sweet, but otherwise plain) cookies.
"Zwo" is used in Swiss dialects besides "zwei". Genuine Bernese dialect uses "zwo" with feminine nouns, "zwee" (pronounced "zwey") with masculine ones.