20M is about as much as I had on RTX4080. 17M is about as much as I had on RTX4080 with a lowered TDP to 200W.
Getting one mobo with 2x PCIE 4.0 x8 and using both cards on one mobo can be a good idea. I don't remember the requirements for CPUs to support each GPU without a performance hit (1 or 2 dedicated cores?, for RTX4090 can be more), but I guess you can find something quite cheap that will be fast enough. Getting ~10M PPD more by switching a mobo is quite a difference ... like adding an RTX4070.