That's interesting, I have to do some more testing then for sure, to see if something else is wrong. I have some various PSUs, including a few CX750 but have not dared to try 2 x Ant's on those. Are you using US source voltage or EU? Although I guess that really shouldn't matter...
Sorta depends on if you want to overclock them. A good 750 PSU should be able to drive 2 Ants at stock clocks (roughly 650 watts output).