The way I would structure the power supplies is have one supply all the PCIe external power connectors on the GPUs and the other supply all the mobo connectors. As long as you turn on the GPU PSU first and turn it off last (or just leave it one - the cards draw hardly any power while off) you won't have any problems. Trying to supply half the mobo and GPUs with 1 PSU and the other half with a 2nd sounds nightmarish.