Can cannot SLI two different Nvidia cards. You can however have two of the same models of nivida cards and sli them. For example, if you have a 2gb 680 by EVGA and paired with a 4gb 680 MSI card, it will work. However you can't pair a 680 and 670. And, you can't pair a 780 and Titan. I would recommend that you just get another 780.