Any GTX970 should be able to SLI with any other GTX970, the only thing that would cause issues is mismatched VRAM sizes and every GTX970 is 4GB (on paper).
As has been said try the FTW+ alone to check it's not duff.
The only difference between a FTW and FTW+ is the new ACX 2.0+ cooler.
As has been said try the FTW+ alone to check it's not duff.
The only difference between a FTW and FTW+ is the new ACX 2.0+ cooler.