I think you should go with a single 770 GPU upgrade.
Advantage of single higher end GPU than two low end GPU's in crossfire/SLI :
1) No microstutter below 30 fps. (they are very irritating)
2) Reduced load on PSU.
3) Guaranteed performance improvement in all SLI/cfx and non-SLI/non cfx gaming titles.
Also, 770 is 19% stronger in performance than the 760. It turns a bit weaker in front of 760 SLI in SLI supporting titles but still has advantages of a single GPU. Even before SLI-ing, you'd miss the quality 1080p gaming exp. on 770. Try getting hands on the 4GB version of GTX 770, it would work fine on your dual monitor setup.
Lets compare.
GTX 770 vs GTX 760 (current performances)
In terms of overall gaming performance, the graphical capabilities of the Nvidia GeForce GTX 770 are noticeably better than the Nvidia GeForce GTX 760.The GeForce GTX 770 has a 66 MHz higher core clock speed and 32 more Texture Mapping Units than the GeForce GTX 760. This results in the GeForce GTX 770 providing 39.8 GTexel/s better texturing performance.The GeForce GTX 770 has 1024 MB more video memory than the GeForce GTX 760, so is likely to be much better at displaying game textures at higher resolutions.The GeForce GTX 770 has 32 GB/sec greater memory bandwidth than the GeForce GTX 760, which means that the memory performance of the GeForce GTX 770 is noticeably better than the GeForce GTX 760.
Therefore, 770 as single GPU must be choosen.