I disagree... The TX650 can handle two GTX 660 with Ease!
I have had a GTX 660 on a i7-2600K (OC to 4.2GHz) powered by Seasonic G360 (specified to 360W - all on the 12V rail) and I've measured about 200W at the Wall @PSU input (translates to about 10% less at the PSU output) with 3DMark and with GPU and CPU at full simultanious stress (furmark+prime95) and used no more than 300W at the wall.
Remember that each GTX 660, only use about 110W each with demanding game load.
Each card only require one PCI-E 6pin, so no need for adaptors.
Corsair TX650 is a good PSU (I have had one for years). Can handle 650W (more than 50 amps@12V) without any instability or Voltage-drop problems (at the output!). With two GTX 660 on a "Regular" modern Quad Core system, you should be hovering at no more than 350W with demanding gaming. That is actually the *sweetspot on the TX650 (*maximum efficiency area)
So you still have headroom for OC and whatever.
BTW. I use two MSI GTX 680 - SLI with a i5-4670K(oc to 4 GHz) on a Fractal Design Newton R3 600W Platinum. The PSU dosn't even break a sweat. Fan starts during gaming load, but is running at low speed - almost silent.
And don't always rely on the PSU wattage-labeling. You can get a Crappy "550W" cheap (Linkworld or whatever) that cannot keep up with a Quality "360W" Seasonic G360.