JeffLewis2 (as opposed to JeffLewis1 :P )

Since the Northwest agreement, you can use Continental vouchers for Northwest flights on Northwest published fares-- and I have gotten a NW voucher for the residual. That effectively converts your CO voucher into a NW voucher.

More tricky but possible: I once tried to ticket a CO res. with a NW voucher and get the residual. CO said they couldn't issue the residual-- but said NW could go into the CO system and issue the ticket at the CO published fare. It took time for the NW agent to do that, and she was coached by phone with a CO agent down the counter, but in the end, she issued the ticket at the CO fare, and gave me a NW voucher for the residual.

Re. coupons, I've run into not being able to use paper coupons with vouchers ("No other discounts or vouchers apply...") but if you have an electronic coupon, like an Entertainment discount, reserve by phone and have the agent apply the electronic coupon to knock dollars off. Then ticket the res. in person using the voucher, and the discounted fare should stick.