no no.
This is how you do it.
Call marriott first and book the reservation with them and tell them to attach the vouchers.
When you check in, deal with a befuddled clerk who can't find both the certs for your stay.
End up checking out with a zero bill.
Log on to discover 1 cert unused.
Post on FlyerTalk about the unsed cert, get a bunch of fuddy-duddies who reply (frankly, typical of MR elites IMHO).
Make a reservation online to use this cert plus a new one using process above in #4.
Need to change the reservation, hit the cancel reservation button online, find that the 2 certs immediately are redeposited in my account, make new reservation in seconds after canceling the old.
Voila.