If the NICs are PCI cards, then they should be able to share interrupts without any problem. PCI defines a way for a card to be identified (by its position on the bus) after the interrupt is raised, so it does support IRQ sharing. That's always assuming that it's correctly implemented in both of the cards.
In general, interrupts are assigned by the plug and play subsystem of the BIOS when the computer first boots up. It's also possible to assign them through I plug and play kernel module I suspect.
Hope that helps,
— Robert J. Lee