GeForce GTX 460M SLI is a solution of two GeForce GTX 460M put together using NVIDIA'S SLI technology.
Check the page of GeForce GTX 460M to know more about its chip.
As usual, SLI relies a lot on proper driver support and may suffer from micro-stuttering in lower frame rates (below 30). Benchmarks indicate the performance is overall, up to X% better than a single GeForce GTX 460M performing by itself but at times (depending whether or not the 3D game supports SLI or in the graphics driver) it performed worse than a single GeForce GTX 460M.
Most demanding games will run at the highest settings with fluent frame rates (30+).