I assume both NICs are internal expansion cards. You probably have them plugged into the motherboard using bus ports that share IRQs. Check your motherboard manual and/or try moving one of the cards to another empty slot. You might also be able to force IRQ settings via the CMOS or through the NIC's configuration utilities. You can check the Device Manager to see if both NICs are attempting to use the same IRQ.
You could also have a driver problem. Be sure to upgrade to the latest drivers from the NIC vendor.
Also, while in the Device Manager, make sure the second NIC is not set as disabled for the current hardware profile.
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