You can use both the 1050 Ti & 750 Ti in the same system. HOWEVER, not in SLI. This means if you play games or use GPU accelerated applications, it will only use the Graphics Card the display is plugged in to. So if you're plugged into the 1050 Ti, the 750 Ti will do nothing to boost performance. You can set the 750 Ti as a dedicated PhysX card but they make very little difference in a vast majority of games nowadays and i'd honestly recommend trying to sell it and use the extra cash to potential get a RX 470 / 570 / 480 / 580 or GTX 1060 instead as it'll offer more than a 1050 Ti + 750 Ti. Hope this helps!
EDIT: Wow 3 answers at once! o-o