Each plug on the cable should be capable of delivering the power it is rated for. While not the usual setup, you should be able to plug both cards into the same cable, providing the cable is long enough to reach.
You are however still bound by the overall capacity of your power supply to deliver current over the 12V rail. If it can't supply the amperage both cards will draw, it won't matter how you plug them into it.
2 x GTX 660 non Ti should run close to 24 Amps of draw on the 12V rail.
I'm using a Seasonic X 750W