Windows 7 or later has no problem at all running two different graphics adapters.
For two monitors, is is simpler to just connect the second side monitor to the GTX970.
While gaming on the primary monitor there is no impact on gaming performance.
I once tried that as an experiment and so no impact.
Probably the only value of a second card might be if you needed to connect more than 4 monitors for something like a stock trading system.
I would just set the GT650 aside to use as a diagnostic part in case of a graphics card failure.