If you have two GTX 560 TI graphics cards, then of course they will consume more power than a single GTX 560 TI. How much? About 2x more. Makes sense?
I never SLI'ed or XFired two video cards together. I prefer a single powerful video card. due to less potential issues such as:
1. Not all games support SLI or XFire. But I assume the more popular big budget games will.
2. Less potential driver issues.
3. Some people complain about micro stuttering in some games.