I think using 2 would be too much, a 6x turnover is more then sufficient for zoas and polys. that would be 330 gph, I would put it in the back corner and ain it to the center front glass, don't forget you also have your return pump this would be on the opposite side, facing the return nozzle and the power head in the same direction would give you a nice flow. buy them both and save the other eventually you will have more coral in your tank that will require more flow.
No get the 750. better choice, remember the rating of these are at the nozzle and not at 12-24 inches away. by the time the water moves at that distance they should be anywhere between 350-450 gph.
Richie's good for the free stuff.. he's really not joking at all. After I get the new powerheads, i'll have 4 korallia's and a red sea wavemaker for people to grab.
personally i think it depends on the brand powerhad your using... if your using simple maxi jet 750 then its not enough. if your using an aqueon, coralife, or hydor, i think two would be better. about one foot down on left and right side aiming slightly upward. if your getting an mp10 and controlling the flow, i think that would be good with a small maxi jet.
imo, one on the left side going across the tank and one on the right side maybe behind your live rock to keep flow where your other powerhead cannot reach.