Each of those graphics cards will top out at about 250 watts at peak load. Your CPU will draw another 125 at peak load. This of course assumes peak load on both graphics cards and your CPU at the same time. At the same extreme, your other components will add another 25 watts or so (some of which will be on the 5 volt and 3.3 volt rails).
Your worst case scenario is 250 + 250 + 125 + 25 = 650 watts on the 12 volt rail. This is well below the 840 watts that PSU is rated to deliver on the 12 volt rail.