The card was previously owned. I've double (and triple and quadruple) checked all the connections. The computer recognizes that there are two identical cards in the system, it just won't give me the option to put them in SLI. Actually, it occasionally works but then stops working on reboot.
Also, thanks for the replies guys. I appreciate it.
Edit: I got it working by removing the drivers and installing outdated ones. Hopefully stable.