You can have both versions reside on the same machine (virtual or real) but only one can be "installed" in /usr/local at a time.  I accomplish this by cloning the various repos into separate directories then sym-linking the one I want to use to "~/op25" prior to running a 'make install'.

e.g.
git clone https://git.osmocom.org/op25 op25.max
git clone https://github.com/boatbod/op25 op25.boat
ln -s ~/op25.max ~/op25
cd ~/op25/build
cmake ../
make
sudo make install

cd ~
rm op25
ln -s op25.boat op25
cd ~/op25/build
cmake ../
make
sudo make install

As an aside, it's very easy to run multiple instances of the same version on the same machine by using different startup scripts and config files.  I do this for my broadcastify streams with very good success.

Graham