How I can install Gimp 2.8 and Gimp 2.9 on the same computer and work with both?
You can always just build it from the git repo (git://git.gnome.org/gimp), but if you really want bleeding edge tech all the time rather than something stable and well-documented, you may want to try gentoo or arch. Either way, you'd want branch master, so:
$ git clone git://git.gnome.org/gimp $ cd gimp $ git checkout master
and follow the instructions in
INSTALL. You can just run the binaries from their location in that folder, no need to actually 'install' them to the root bin and lib directories.
If you want two conflicting versions of the same software accessible to aptitude, though, you will have to do some mucking about with package building.