Yes you can SLI the 960.
I personally do not think SLI is worth the price for the performance gains and potential issues, but it should work fine with your setup, and midrange cards like the 960 are generally where SLI is most worthwhile.

My point is I generally find it more cost effective to just get a single newer/better card than spend the money on another older card, more bang for your $$$ generally. As you can see here: https://www.gamersnexus.net/guides/1830-asus-strix-gtx-960-sli-benchmark-vs-980-970
SLI 960s are often beaten out by a single 970. You can get a 970 on ebay for $150-$175. A 960 goes for $80-$100 on ebay. So even if you sold your 960 for $80 and bought a 970 for $175, cost would be $95 and you get better performance, less power usage, no SLI issues to deal with. If you can get a 960 cheap go for it, otherwise I think there are better options.
 
Also re: Battlefield, from linked article: "From our testing, there's almost no value in a second GTX 960 for Battlefield 4."