If one's a PCI and the other's an AGP, there's no reason why you *shouldn't* be able to have them both in at the same time. Bear in mind I've never done it, but I'd imagine that the worst that can happen is that your computer will kind of flip out when you boot it up. But two boards in one machine isn't unheard of.
My Magic 8-Ball, which is pure opinion, says "Go for it, let us know what happens."