May be problematic, due to some shared files and settings between drivers. Much easier to do with Kepler GPUs, which had driver that still supported cards like 7800GTX. That said, newer drivers may add additional bugs even on old video cards and Nvidia weren't good in long-term support of pre-DX10 hardware (so did AMD with old ATi cards).

Having two distinct Windows XP installations (on separate hard disks/SSD) in a single machine and booting from the one you want using the BIOS' boot menu option would allow each GPU to have its own Windows XP installation .

It is not exactly related to running 2 gpus both under windows XP but has anyont tried running virtual machine when having newer and older gpu in same system and doing gpu passthrough of the older gpu to a windows XP virtual machine for compatibility reasons?

Over the years I've built a few weird mixes, usually for current need and not retro.. Multiple ATI/Amd's usually work well together. I've used vm's for years now and pass a gpu. I've had 4 gpu's in a system. PCI-e generations can give issues depending on the motherboard. PCI cards can work well, later ones with bridges can cause issues. Cabling and lower lane widths work fine. Using a common backing files and layering images can run two instances on two gpu's simultaneously.

Lots of possibility. Unfortunately I've found a working combo doesn't necessarily migrate to another board.

so more or less the nutshell, no solution that is smoother than the other, easiest with less problems is a dual boot xp machine?
more a problem between drivers then?