A web search using: gtx 660 sli review should provide a link to a review that will tell you how much power (A.C. watts) your power supply draws from the wall outlet. You should not draw more than 80% of the rated power output of your power supply for an extended perior of time, such as PC gaming. Your TX650 psu is 80 PLUS bronze or 80 PLUS rated and with higher loads would be about 82% efficient. A little math:
650 watts (D.C.) x 80% = 520 watts
This is the maximum load that your psu should provide for a sustained period of time.
If a GTX 660 SLI review states they their test system draws 550 watts (A.C.) then 550 watts x 0.82 efficiency = 451 watts, so you will be about 70 watts under the maximum safe rating for your power supply.
Note: the Nvidia Ti video cards draw more power than the non-Ti cards.
From what I've seen, the GTX 660 standard cards require one pci-e power connector for each card.
If you don't overclock your cpu and video cards you should be fine.
If you overclock, your psu might eventually be over taxed if you have an older psu.
The TX650 has been around since 2008.
If you have an older model it might be a good time for an upgrade.
Your current psu is probably more than adequate for an SLI setup.
The information from eXtreme Power Supply Calculator should give you a good idea for the optimal power supply wattage.
I found an XbitLabs 660 Ti review that shows the power draw while running Metro 2033:
from: GeForce GTX 660 Ti 2 GB in 2-Way and 3-Way SLI Configurations . Page 7 - X-bit labs