You can't. You can either use one or the other. As for SLI, you must use 2 or more of the same GPUs.
For example, SLI for GTX1070 would be 1070 x 2 or x3.
SLI for 960 would be the same. 960 x 2 or x3.
Your mobo also needs to be SLI compatible.
ok i figured this might be the case thanks