Yes thats fine to use one PSU to help another out as long as you keep them on separate blades. So the 1300 will power 5 blades, the corsair 3.
haha just realized im then right on the edge with the 1300 W PSU. Maybe not as close as with the 850.
Am I understanding right that you link the boards together? So basically I'd have 4 boards from one assembly and 1 board from a second assembly linked and powered by the 1300 and the remaining 3 boards on the second assembly linked and powered by the 850?