I would call ahead and ask, or ask when you get there, but be aware that your service will likely be cruddy there if you let them know ahead of time you have a voucher.
When we went, our waiter was very unhelpful once we presented our voucher... I can't even tell you how rude they were. I'm not the only one who's had the same exp there. My best advice would be to call and speak with a manager ahead of time (get name), then don't mention it again until your check it presented. If your waiter refuses them both, then you can present your conversation with the manager and ask to speak with him.
My exp there has not been good... they have not been flexible in the past, or kind to those using GC's or vouchers... so I would not expect them to okay it, but check ahead of time to make sure. That's my best advice =) Good luck!