It's nice that they are using a relay to control the slave PSUs. That is better than the usual "connect all green wires together" method.

However, while you can use it like this, you cannot plug one device (video card, big BTC miner) to two different PSUs without special load balancing circuit (like it is done for dual PSUs in servers etc). You can however use the slave PSUs for video cards (one PSU per card) or separate miners (one PSU per miner).