Cross feeding power to your GPU's via two PSU's should never be done !

If one of the PSU's develops a fault and stops working for some reason because one of the connectors cables has melted and started a fire or one of the GPU's has a burn out (catches fire) it's going to cause a feedback loop to all other devices and kill them if your unlucky.

If you want to mix and match this you risk back-feed/run-away into the motherboard and will probably break stuff and/or set your house on fire too.

Power your devices the right way the first time.