I probably can’t help you very much here.
But:
this, to me, looks like the system is up and running, only the graphical interface somehow failed to start
meaning:
you can use a TTY
to assess the situation
… gather logs, for instance
and, probably equally important to people who CAN help
provide hardware info
look for and find the tread(s) here on “how to provide good information”
it starts with inxi -f (or similar) output
and has some more hints for you
recognize that you sit in front of your machine - no one else here is
we rely on information
what was, what changed, what is now - for example
as much as you can muster (after finding and then reading the mentioned thread)
so that people here do not have to tease needed info out of you bit by bit.
Are these two GPU’s supposed to use the same driver?
Or two distinct ones, one works for this, one for the other?
for example
You could, presumably, also go back to a working system (by switching back the GPU’s)
and from there research what is needed to make the new GPU work and what changes might be needed.
You should check xorg.0.log before doing anything. If the issue is in wrong driver loading, it will be easier to backup and delete xorg.conf and xorg.conf.d.