twofer vs coupon

twofer and coupon both are nouns.

Word	Noun	Adjective	Verb	Adverb
twofer	Yes	No	No	No
coupon	Yes	No	No	No

As nouns, coupon is a hypernym of twofer; that is, coupon is a word with a broader meaning than twofer:

twofer: a coupon that allows the holder to purchase two items (as two tickets to a play) for the price of one
coupon: a negotiable certificate that can be detached and redeemed as needed

Other hypernyms of twofer include voucher.

twofer (noun)	coupon (noun)
an offer of two for the price of one	a test sample of some substance
a coupon that allows the holder to purchase two items (as two tickets to a play) for the price of one	a negotiable certificate that can be detached and redeemed as needed