Also keep in mind that the S7 "power requirement" spec is "at the wall" using a 93% efficient PS - so the actual draw would be a little less.
However, trying to power 2 hashboards from an 850 would be very marginal at best.
2 hashboard on that Seasonic 1000 and the third + miner control board + fans on one of the 850 however should work fine.
This is correct and the same as my proposition.
Depending how it's clocked (more specifically, what version of miner) it should be possible. I've had several in hosting that have been running off a pair of Dell 750W server PSUs, two boards to one supply and one board + controller to the other, for about a year with no issues except on the hottest days. Won't handle some of the new very-hungry S9/T9 but did stock S7s.
It's also not too difficult, if you have the right tools, to undervolt an S7 for greatly improved efficiency. Low enough to run off a single 1000W PSU and still get around 4TH.
This is good point, I used to have great time with underclocked S7 and Enermax Revolution 87+ 1000W PSU.
Sidehack is the magic man when it comes to mining hardware and he has a topic for this "underclocked S7" subject:
https://bitcointalk.org/index.php?topic=1504228.0