I guess I will have to check the cables as well. I really think the problem is the bridge between the PSUs, it's running perfectly for a few days, but if the energy fails or I try to remotely restart it, then the problem starts again until it gets stable.

I will try with a more powerful PSU for the main one (mobo + 2 gpus), because if the cable heats a lot, it may overcharge it just a little, enough to reduce it's efficiency to a level close or lower than the actual power output, reseting the system. Since the PSU is almost on its power output limit. It would explain the fact that the system gets stable after a few tries, as the components / cables will cool after some stand by period.

But I guess I had two problems, one was with the "generic add2psu" that failed and caused a collapse over the system, other is related to overcharge of the capacity of the main psu (under tests).

Thank you all for your contributions, feel free to enlighten me more if you have anything else to say!!

Ah, yeah I know it's bad enough to use 3 PSU in a row, but server PSU where I live is too costly as well as powerful PSUs, it goes for more than $400 a 1000W PSU.