When working with Nvidia cards, SLI would be the appropriate term, crossfire is for AMD multi-GPU set up.
SLI will work as long as the gpu chipset is the same. With two differently clocked cards, they will run at the lower cards clock speed in SLI
I did something similar: two of the same gpu's make by MSI and GIGABYTE.
You can either SLI them and let NVIDIA adjust the clock speed OR use EVGA precisionX to adjust the clock speed of the slower card to match the higher card and the enable SLI. Use GPU-z to verify matching clock speeds.