It should work ok as long as you make sure the cards are happy enough. Some cards like to share interrupts etc whilst with others you need to make sure they definately dont share !
I presume your trying to make the PC into a PVR type device (watch one, record one) so it shouldn't load the PC too much more than a single Mpeg decode stream. If your actually trying to view two at once then the PC needs to be relatively powerful to cope.