Multiple Power Supply Adapter Sync Start Card Dual PSU Connector Jumper Connector with Sata 2 Pack

That will let the psu turn on whenever plugged in, meant for testing. you’ll want a tiny board which two psus plug into and one signals to the other to turn on. Like this… https://www.amazon.com/Thsion-Synchronous-Multiple-Adapter-Connector/dp/B08F9WGLP2