I think all you need to do is make sure you get a 570 with the same amount of vram as yours (i.e. not a 2.5GB version) - the pair of cards will run at whichever set of clock speeds is slower.
Ideally you can get another EVGA SC model, but I don't think you have to.
Other factors are making sure your motherboard supports SLI and can do so in PCI-e 2.0 x16/x16, x16/x8, or x8/x8 mode (not x16/x4) and making sure that your power supply is good enough to handle both cards.
Thank you for the clarification. I'll be looking for another EVGA 570, that's for sure. I am pretty sure my motherboard supports two 570's and is sli ready. PSU should be good to go. Being my first build I don't remember what exactly I have for a few components, but I do remember researching and buying things that would set me up for sli at some point.