Personally I'm not a big fan of SLI'ing any card less than a GTX
960 970. I recommend selling the 960 and buying either a GTX 1060 or 1070; depending on your budget and build goals.
Addendum: Please be aware that GTX 1060 does not support SLI, while the 1070 does. Also, once you upgrade above the 1060, you may "possibly" experience diminishing returns due to your CPU limitations. If you choose to pair it with a 1070 or above, then you may want to investigate if that will be the case.