No, use PSU A to power MB and x3 GPU risers and pcies, then PSU B the other GPUS risers and pcies. Power A first, then B.

Either use an 'add to PSU' connector to link the two, or simply short the MB connector on psu B and switch it on second.

That way if your two PSUs outputs vary at all, it won't be your gpus in the middle.