What CPU do you have?
How much vRAM have those GTX 750 Ti? You remember that the vRAM available in Cycles is not the sum of both. You will have available a total vRAM equal to the card with the lowest vRAM.
2GB vRAM is currently little to Cycles/CUDA.
I had asked about your CPU. If you have an i7 CPU with many threads, this may be faster than the two GPU options that you are showing.
I guess you would not have advantages over the CPU with neither of these GPU options. Also if you have an i7 2700k, your CPU is overclockable and can improve the times.
You try to get at least a GTX 970.
If your scenes will not use SSS, hair, complex transparent/translucent materials with emission node plugged in Volume and volumetrics in general, then the GTX 960 4GB can be more than twice as fast as your CPU (not overclocked).
What were your CPU render time with the two scenes that I have shared in the thread above?
Sorry, I forgot to say that in addition to choosing CPU under Device, you must choose the size of Tiles: x=32, y=32 under Performance. You should see 8 small tiles working simultaneously.
Anyway I think that you get a weird render time with the second scene. Did you use the scene from message #2 in the other thread, right?