You do not need a 1200 and 1000, thats overkill. Full power draw of a 6x280x system is 1665watt. Get 2 decent PSU's to cover that much wattage with a little overhead.
I've been wondering about this overhead. Is it safe to run a PSU at full capacity 24/7 non stop for a really long time? How much leftover wattage (if any) do you need for a stable rig temperature wise? 100-200 watts? 10% of the watts?