Difficult question! Depends on multiple factors.
VRAM: GTX 670 has 2 GB, SLI 670s 2 GB as well (VRAM isn't doubled in SLI or CF). Might be an issue if the texture resolution in the game is really high
Performance: An overclocked GTX 680 can dip in the low 40s at 1080p with the highest settings:
http://www.xbitlabs.com/articles/gra...roundup_9.html
Given that Rome 2 will probably scale well in the high end (CA said so), a single GTX 670 might not be enough in all situations.
Price: A 780 costs around 550 Euros, a second 670 only about 300. From a value perspective, the second 670 is a way better deal.
Microstuttering: In my experience (580 SLI, Titan SLI), microstuttering becomes an issue below about 50 fps. Personally I can live with it since I mostly play with 60 fps anyway, but if you want to crank up the settings and get like 40-50 fps, it might not be perfectly smooth.
Overclocking: The 780 seems to be an excellent overclocker. 25-30% should be no problem. As far as I know, the 670 isn't that good of an overclocker.
My suggestion:
Go with the 780, overclock it (or not
). Due to the 3 GB VRAM and the 50% higher bandwidth it definitely is more future proof than the 670 in SLI.