That's correct, I have 2 x 850W power supplies.

1st power supply supports 2x RTX3090 + PC (motherboard, cpu etc.)

2nd power supply supports only 2x RTX3090

They are connected to the same power strip. So they are always turned on or off at same time.

Don't forget, you need a power supply jumper for stand alone PSU.

Something like this: https://www.amazon.com/SDTC-Tech-Starter-Without-Motherboard/dp/B07X9SVB5K/ref=sr_1_3?dchild=1&keywords=power+supply+jumper&qid=1630004705&sr=8-3

Or you can short the pins yourself.

You can use something like this https://www.amazon.com/SDTC-Tech-Starter-Without-Motherboard/dp/B07X9SVB5K/ref=mp_s_a_1_4?dchild=1&keywords=24-Pin+ATX+EPS+PSU+Power+Supply+Jumper&qid=1618600927&sr=8-4 Use this on your other power supply that will be used for the 1080. When you boot your PC turn the 1080 PSU on and it will work as normal.