It has to be one motherboard that can put at least 2 power supplies, yeah I know that you can do a diy here but for maximum safety, is better done internally. A great motherboard is the asus b250 mining expert, you can put 3 psus and up to 13 gpus without issues.
I have had bad experiences with the B250 Mining Expert back in 2017/2018.
I like using a 500-700w ATX power supply for the MOBO + 1 or 2 video cards, then HP server power supplies for the remaining video cards. That is the cheapest combination while still having Gold/Platinum efficiency.
And why use 500-700w ATX for the motherboard, if an inexpensive, good quality power supply of 300-400 watts is enough, and connect the rest of the video cards to the server power supply?
Or does your server PSU not have enough power?