So short answer: you might be able run it - but you wont have any performance benefit.
Long answer: your CPU and GFX transfer a hell of a lot of data to and from eachother, to render your display (calculate pixels etc). So now, when you slot in a second card (for explanation purposes you will have a Geforce 960TI slotted already), now you add a second one (provided your MOBO supports SLI).
Now your CPU (depending on which CPU), has the potential to talk to the GPU's over 32 PCI-E lanes, instead of just 16.
Imagine a highway with 8 lanes in each direction - which is stacked with cars bumper to bumper, add 8 more lanes in each direction, and you can fit more cars.
Thats pretty much as simple as it gets.
P.S. to make use of all 32...