Even a GTX 980 Ti or Titan won't be able to run a single 4k monitor at 60 FPS. GTX 960 is a mid range video and expected to run games at good settings @ 1080p.
OP didn't say anything about running games on those monitors.
OP, if all you want to do is run the two 4k monitors for non-gaming use, then YES, you can connect them to your 960's DisplayPorts and it'll handle that just fine.