As an FO you will be able to upgrade for free at 24 hours before departure if there are seats available. Or, if you can use this cert for travel with a higher level medallion and have you both on the same PNR, you can both upgrade at their window (GM=72 hours, PM/DM= at time of purchase). What I'm not sure about is if you would be able to both be on the same PNR with one of you paying with the cert and another with cash. My guess is that the initial response would be no, but maybe you could push it with a supervisor.