I had an average level (Bronze rating) 850 watt psu that I was going to use to run a pair of 290x's but I didn't get a second card for other reasons in the end.
The thing is AMD recommends using a 600 watt psu with one Grenada chipped card.
We know Grenada uses more power at load than Hawaii did from reviews that test this so why would you risk it ?
If something did go wrong you'd have to lie about your psu or risk being told it's your own fault for not following the guidelines.
Thats AMD being over cautious, most people who post here and places like this know what is written on the box of the PSU is not necessarily what you get out of it, it may well say 600 Watts on the box but what you get from the 12v rail is actually only 350 Watts if you end up with something like one of those CiT PSU's, most people don't know this, not everyone who builds PC for themselves post here.
The 390 uses 220 to 250 Watts, put two of them together its 500 watts, a true 750 PSU is plenty.