After doing a few tests on my system with and without SLi, running 2 770's that SLi is not currently supported (or at least isn't running) in the actual released client. It is however supported and running well in the Character Benchmark.

Test Results:

System:
Haswell 4770k 3.5GHZ OCed to 4.6ghz, water cooled at 20-30 degrees C
16gb Corsair Vengence Memory at 1886MHZ
Game installed on 250gb Samsung SSD
2 x Palit 770 GTX GPU's running 326.80 drivers.
Windows 7 Ultimate 64bit

Main Client (FPS is average) and running at 1440p Max Settings
1 GPU Installed: 78 FPS & 45 FPS during busy FATE's
2 GPU Installed, SLi Disabled: 79 FPS & 43 FPS during busy FATE's
2 GPU Installed, SLi Enabled: 76 FPS & 47 FPS during busy FATE's (both GPU's only displaying 48-58% usage each suggesting SLi isn't working)

Conclusion: All pretty much the same. The difference is normal and suggests that the same amount of GPU power is being used by the game.

Character Benchmark (run at 1080p Max Settings)
1 GPU Installed: 12022
2 GPU Installed, SLi Disabled: 12097
2 GPU Installed, SLi Enabled: 18912

Conclusion: SLi is indeed supported by the benchmark with 80-100% GPU usage when SLi is enabled.

I hope this helps answer the question at least with SLi on the 7 series GPU's, obviously I cannot draw any conclusions in relation to Crossfire.