Make sure not to get a MSI as it blocks ECC. I believe most other brands do unofficially work with ECC but I cannot guarantee anything.
If you want to passthrough two PCIe slots (both working in x8 mode) you need a motherboard with a X570(S)...
You can find this information by checking the block diagram published in the motherboard manual or on the web.
Whether it operates with two units also requires checking the manual to see if the IOMMU can be separated.
ex...
I'm searching a B550 board with ECC support and opssibility to passthrough two PCIe slots (GPUs).
How do you find out, that the secound PCIe is connected to the CPU?
And can all boards with "AMD 2-Way-CrossFireX" passthrough 2 PCIe slots to...