Which PSU powers the server in a redundant PSU environment?

Hi,

i have a T620 and a T430 with both redundant PSUs and two UPSs (1x 3000VA with NMC and 1x 1000VA without NMC)

I  want to configure it this way:

The primary PSUs are plugged in in the 1000VA PSU and the secondary in the 3000VA.

Once the 1000VA is empty the secondary PSUs will use the 3000VA UPS. If the 3000VA UPS has low battery the servers should shut down.

The shutdown part is already configured.

My question now is the following: Do the redundant PSUs split the needed energy 50/50 or is there a primary PSU which powers the server on its own?

If there is a primary, which is it?