Thank you Wile!
Now I have both, development and pro, versions installed following your suggestion in the post Sourcing root for all users. I have the following lines in my .bashrc file, and switch between the versions as required.
alias root_6.23.01_b3ecdf6='source /opt/root/dev/bin/thisroot.sh'
alias root-dev="root_6.23.01_b3ecdf6"
alias root_6.20.04='source /opt/root/pro/bin/thisroot.sh'
alias root-pro="root_6.20.04"
Both working fine when compiled with:
cmake ../root-6.20.04(OR root_6.23.01_b3ecdf6) -DCMAKE_INSTALL_PREFIX=/opt/root/pro(OR dev) -Dcling=ON -Droofit=ON -Drpath=ON -Dsoversion=ON -Druntime_cxxmodules=OFF -Dpython2=ON -DPYTHON_EXECUTABLE=/usr/bin/python2
However, adding "-Dfortran=ON", the compilation fails for both the versions.
I also had to set "-Druntime_cxxmodules=OFF" as suggested by @eguiraud. Please see: Error Compiling ROOT 6.20/04.
Finally, I would like to point out that the issues of missing (or rather misplaced) libraries as mentioned in the IMPORTANT NOTE in Python3 and ROOT 6.23/01 and Different INSTALL-directory structure for different installation methods arise when -Dgnuinstall is set ON.