Hi everyone!
Just created an account to report here that I had bought the aforementioned aliexpress card (here) and received it today.
I can happily report that I succesfully managed to split the 4 controllers and assign 2 to each of my Windows VMs. Only had to enable PCIe ACS override: Downstream to split the IOMMU groups.
Was rather cheap, took a while to arrive, but works beautifully.