With video cards the hardware does not add together , so a 4gb card in SLI with another does not give you 8gb to use, same goes for the 256 bit and 384 bit.
However putting two cards like the 760 I SLI will increase performance, not to the point where it doubles the FPS but almost.
Benchmarks have shown that two 760's in SLI will perform the same or better then a GTX Titan depending on the game and the resolution.
So you would have an excellent gaming rig with two 760's.