Gentoo supports slots; which allows for concurrent installations of things like desktop environments. Not sure if it's configured that way right now though.
If you use BTRFS you could install gentoo on a subvolume and boot into it when needed too.
@arran4@voracread With some specific packages there are conflicts between different versions, so you can only have one at a time installed even if they have different slots. I think a few of the core KDE packages are like that. I know that I had to remove some parts of KDE 5 when I installed KDE 6 on my Gentoo system.