Don't use cheap PSUs, and don't use dual PSUs on one system if you can possibly help it.  Unless you're just making use of some high quality, lower wattage PSUs you had already, there is absolutely no reason to be using more than one PSU in a rig.  I have never seen a pair of PSUs that cost less than a comparable wattage single PSU of the same quality and efficiency.

If you still insist on using dual PSUs, keep them electrically isolated from each other - a GPU and riser combo should only receive power from a single PSU, never split.  Same for the motherboard.  Use only powered, USB-connected risers -- never the ribbon cable ones.  Failure to isolate can kill even the highest quality power supplies.

With a 7-GPU rig, you're making $35+ per day.  Saving $50 by using a cheaper PSU is pointless if you end up having to RMA it or wait for a replacement.