because not all cards require two 6 pin connectors, some only require one... try a molex adaptor if you need it... btu in general i'd imagine if the psu only has 2 6 pins i'd question it actually being able to support two 460's
A couple of years ago there were very few if any cards that required 2 6 pin connectors so the power supply manufacturers didn't anticipate the need to put the extra connectors on there. The adapters work fine if you have enough watts on the 12v rail. I've got 2 465's in SLI running on a 620w PSU, one uses the 2 6 pins and the other uses the molex adapters and I haven't had any issues yet.
I am rather curious as to why someone would even bother to ask this question when all one has to do was look up the base power consumption of this card. 160w on reference clocks (with margin for give or take) with a max out over 200w. If you see a power connector it doesn't mean that it optional like a usb port or a serial port but is required for the normal operation of the card. Two power connectors equal that you need to hook up two power cable to ensure that the card works. Common sense seams to be all out the window these days no matter ware one is in this third dung heap from the sun.
The 5770 in general uses less power than the GTX460 but however it doesn't compete performance wise and putting a second in crossfire defeats the purpose when efficiency is in mind.