multiple GPU's 750ti + 980 gtx

Solution
Yes, it's possible if your PSU and motherboard can handle it, although it might not really boost your performance, and it'll also block airflow and generate more heat.
You mean a GTX 980, which is currently not released, or the ancient 9800 GTX?

Either way, not really worth it.

A single GTX 750 Ti would outperform two 9800 GTXs in SLI.
And a single GTX 980 would be so much stronger than a GTX 750 Ti that it wouldn't be worth using as a PhysX card, let alone two GTX 980s.