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 Lanes, you will have to have a CPU that supports 32 Lanes, Most CPU's only support 24 (so 16 from your first GPU and 8 from your second), which still is a big performance boost, usually you can consider around ~50% extra performance for the second card.
Copy past specs from a I7-2011 CPU:
Details
Core Name Haswell-E
# of Cores 6-Core
# of Threads 12
Operating Frequency 3.5 GHz
Max Turbo Frequency 3.7 GHz
L2 Cache 6 x 256KB
L3 Cache 15MB
PCI Express Revision 3.0
Number of PCI-E Lanes 40
Hope this answers your question.
Facebook
Twitter
Instagram