Two identical NICs not being recognized
We have tried with two identical Realtek NICs and then with two identical Intel NICs, and only the first one is being recognized (re0 for example). The second NIC lights up but the link up is never recognized and it doesn't show as a valid interface. It gives a "network interface mismatch" before it shows the valid interfaces.
What version of pfSense?
nanoBSD or full?
The second NIC lights up but the link up is never recognized and it doesn't show as a valid interface.
In what context does it "not show": GUI: Interfaces -> (assign)? Console Assign Interfaces dialogue? Somewhere else?
It gives a "network interface mismatch" before it shows the valid interfaces.
In what context: startup?
Are the two interfaces on a single card or two separate cards?
Have you tried using a different slot for the troublesome interface?
Swapping the cards may also be helpful to try. If the problem follows the card, it's the card, if the problem follows the slot, it's the slot/motherboard.
It's very unlikely that it's a software issue, unfortunately.
Seems far more likely to be a motherboard/bios problem since you have tried pairs of Realtek and Intel NICs.
Have you tried one of each?