This might be a lot less complicated if you used one of the Solus instances to host the second instance as a VM. VirtualBox (in the repository) allows you to share a folder between the host and the guest. It would seem that you could share the host's /home folder with the guest instance.
I'm not sufficiently interested in trying that as more than a thought experiment. If you really want to make it happen, I think that could be a way you might make it work.
Another approach might be to install both DEs into one Solus instance so that you could switch between them at login time, but not run both at the same time. That way, they could share everything. Dunno if that matters, since I can't think of any practical reason to do this in the first place..