because you still get the processing power of both cards
ie - the shaders, the texturing units, the raw processing power etc...
you just dont get the extra memory because only 1 card is actually outputting the frames and so you can only use that cards framebuffer, thats what memory is, its the framebuffer.
The GTX650 ti has 768 processors, so you will get 1536 processors by running 2 of them. (incase you didnt know a GPU is essentially hundreds of highly specialized cpu's running in parrallel)
1x GTX650ti = 2gb ram + 768 processors
2x GTX650ti = 2gb ram + 1536 processors