Setting aside LoL being a very famous, but DEFINETLY NOT enjoyable game (horrible community, impossible PVP unless you're rank 30 and own a decent set of runes, while specializing in a single champion), any game can support SLI; it's just a matter of whether the game addresses it or not.
On an oversimplified example, it's like a quad-core CPU running a dual-core optimized game; it'll run, but it'll likely be fully loading only two cores and using the extra two for offloading extra or OS-related stuff.
You could say pretty much any game sporting full DX10 support and above likely supports SLI/Crossfire as well. If it actually gains any advantage from it, however, is an entirely different matter...
LoL doesn't need it anyway. 60 fps or 300 aren't gonna make a difference on the top-down run-of-the-mill MOBA loved by half of the world. You need low ping, hand speed, mouse accuracy and a good amount of eye-hand coordination.