voucher vs twofer

voucher and twofer both are nouns.

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

As nouns, twofer is a hyponym of voucher; that is, twofer is a word with a more specific, narrower meaning than voucher:

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

Other hyponyms of voucher include book token, luncheon voucher, meal ticket.

voucher (noun)	twofer (noun)
a negotiable certificate that can be detached and redeemed as needed	an offer of two for the price of one
a document that serves as evidence of some expenditure	a coupon that allows the holder to purchase two items (as two tickets to a play) for the price of one
someone who vouches for another or for the correctness of a statement