A few things. First is the good card disabled while you are using this other card? I suppose you have exceeded your number of connections to your server if for some reason it is still making connections to the other card.
What you need to do is look at what speeds you are linking at. Go to Device Manager -> Network Adapters -> the "bad" card -> right click and select link speed -> under Speed and Duplex you could choose 1GB/s and Full, but it is always best to select Auto Negotiation and let each end (NIC and switch) choose the fastest link. See if the "bad" card is set at its fastest speed. While you are there, you can right click on the other card and select disable if it isn't already.
But, to the real issue. I have probably owned at least 30 computers. NONE of them ever had more than one card. There really is no advantage. NIC cards seldom go bad. I don't even have two cards in my $4500 server. You could keep another card on hand. It will take you all of five minutes to install it. That gives you the advantage of an extra card for all of the computers rather than having to have an extra card in each.
The only thing I can think of is the limit of 10 connections, although I have read many places where you can have more especially with registry hacks. Remember connections to network printers and scanners all count.
If you reboot the computer, are you still limited, but one computer that couldn't connect can, and one that could now cannot?
