you can set the other one as dedicated PhsyX although i don't think there is gonna be a significant improvement. So better stick with your single 1050ti
They will work, no bottlenck... but the thing is that they would not work on parallel or for the same purpose... you need to have the same chip so you can use them in SLI
If the lower end card is relatively close to the main card in spec, using the lower as a dedicated physx card isn't a bad idea for physx heavy games like Arkham Knight. You won't see your frames drop as much with interactive fog or when there's a big explosion, etc.