add2psu looks like an overpriced gimmick.  Molex connector to trip what appears to be a pbc mounted relay.  Relay shorts the "on" wired for the ATX connector, thus turning on the other PSU.

There's no way it can load balance.

Also, you need to factor in efficiency of the PSU.  if it's rated/tested at 90% efficient, then your 71a can only sustain 62.9a under full load.
Also, powering up may spike the current draw above that, making your PSU trip it's internal breaker and shut down, or blow a fuse/burn out something if it doesn't have an internal breaker.

Sounds like you need a better power supply.  I'd go with a 1250 seasonic to power 4 GPUs, and use one of your existing ones to power the remaining 2 + system.

What power supply do you have?