I don't think so. It should be possible to install the older version in different prefix (e.g. `./configure --prefix=/opt`) but I don't think AUR supports multiple slots (like portage does) out of the box.

My cursory search on the interweb did not yield any promising results on the Arch wiki either. Happy to be corrected, of course.

> Would the AUR let me have the current Krita and the old one installed simultaneously?

I doubt it.

> My experience with repo/package models suggests it very much would not.

The Nix package manager would. However, I have no idea if their packages are kept up-to-date like that.

If you comment out that your aur build provides and conflicts with krita and install it under /usr/local or some such you can do parallel installs.