GTX 750 Ti does not allow SLI (dual) mode. Nvidia have enabled that feature exclusively on higher models only. Even if you could dual operation, 500 W would be plenty enough. You could even SLI (dual) two GTX 970 with that, if you psu is not bad quality one. But I am not telling anything new here, just confirm what others already said. Best is to sell the 750 Ti and buy a GTX 970. That is much more improvement than what you could achieve even in theoretical SLI mode of 750 Ti. I did the same and man... both cards are fantastic for the money. Btw, which exact psu model do you have and how do you come to 900 W with a graphics card like 750 Ti? Even 400 W would be enough (I had 400 W previously).
Edit: And something else.
Adam_10 :
Thank you for that, I was going to buy a SSD to run dual 750 as I have been advised to do so
I don't understand this. What have an SSD to do with dual (SLI) operation of graphics card?