I'm not sure but I think you need the same GPU to SLI. Brand is not a requirement though. So for example, you have an ASUS and an EVGA 560TI. You can SLI those. Not the 560Ti and the 550TI however.
2 x 560TI performance is almost similar but significantly more power-hungry and heat producing. Its also going to be more expensive. If you can afford that, just get a gtx 670.