If one of the PSUs is just powering video cards you do not need to reset it when you reset the PSU powering the board.
Thats true, but at my place Internet goes down at random times and I have a Orange Pi shutting down the system when that happens so, control of the second PSU is needed in my case unfortunately.
FYI, the best way to link two PSU is to simply connect the PS_ON line (I believe grey?) between both PSU's. They will both get the power-on line pulled low at the same time... so it should fix the rebooting/power off issues.
For issues with the PSU not turning on fast enough; just be sure the PSU with the slower switch-on time is connected to the mainboard of the PC motherboard (if using in that config).
I always like to keep the circuits isolated as much as possible(paranoid maybe) thats is one of the reason I went for this setup.
Could you please specify the cable pinout from relay?
it is possible to make 2 slaves psus?
The pin out is as follows
4------5
| |
2--3--1
Pins 1 & 2 is the main circuit and pins 3,4 & 5 is the secondary circuit
when there is no power between pin 1 & 2, pin 3 is connected to pin 5, but when there is power between pin 1 & 2, pin 3 is connected to pin 4
So connect pin 1 & 2 to the main PSU's 12V & neutral and pin 3 & 4 to pin 15 & 16 of the secondary PSU.
To answer your second question, yes you can but I wouldn't recommend, I like to keep the circuits isolated as much as possible, as adding the third psu is simple, repeat the above and you can control it.
Hope this helps
Regards,
BLife