While you can run two different cards together, they wont work together as you want. You want SLI for nVidia or Crossfire for AMD, which is where they share the load.
For two different cards, you can set one to do the Physx calculations, while the other does the general work. Not sure this would be very beneficial unless you play one of those rare games that includes Physx.
Also, nVidia does not allow SLI on any cards below the 1070, and they do not have a connector for the required bridge. AMD however, allows Crossfire on all their cards, but they use the PCI-e pipeline so that no bridge is required.
In short:
nVidia: SLI = Running two of the same card to increase performance, no allowed on models below 1070
Card run two different...