I can confirm that it is possible to use both PS/2 keyboard and mouse with a motherboard that only has a PS/2 combo port (using a splitter) albeit with a narrow combination of hardware and software
It’s been something I’ve looked into for a while and whereas it definitely worked in the 90s it was unclear whether it is possible with newer hardware in the 2020s. Perhaps it works fairly reliably but those users don’t report it. I could find very little info on it working with Qubes.
The four factors are the splitter, the combo port hardware, the bios software and whether the OS supports it
My hardware is MSI Z790-P and splitter off eBay (it seems the Dell one is rather reliable although mine is not Dell).
This did not work with the MSI bios, it only recognized the keyboard. However Dasharo bios allows it to work on Qubes