I think they get assign a number according to the oder they are installed.
If both are present on install then the PCI route would determine which is first.
The irq is assigned depending on the card (hard coded or PnP) but can be set in the bios PCI slots for PnP devices.
Try reversing the crads in the slots, leave out the second one, and see if the reverved card is now etho.
Add the second card in the same slot as the other was in orginally, and it should be eth1.
The 0 and 1 are not ports, they are device naming conventions.