You are probably asking for problems if you "combine" the two. There could be issues with shared directories, or port conflicts, depending on exactly how the installation/set-up is done. Given that V12 is still in Beta, I would want to run it on a completely independent piece of hardware, you don't want to risk affecting a working system. You will then be sure that any issues that do arise (and at this point, there will do doubt be a few) are not caused by a conflict with the other version. Version 12 Beta may have to be completely removed and be re-installed once it it ready for final release.
Just my opinion, other may see things differently.