What other option do you have instead of 2xGTX 960? If you're looking for performance similar to the 960 SLI, look for a used GTX 980/1070 or new 1650 Super/1660.
The GTX 960 in SLI will only reach up to about a GTX 980 in performance assuming the game supports SLI and is programmed well enough to take advantage of more than one card. You will be limited to whatever total vram is on both cards and if one has 2GB and the other has 4GB you will be limited to 2GB vram. They will also be dumping out a lot heat into the case and require more power than the GTX 1070.