This is the same problem addressed by the ancient, universally known tool named virtualenv. How are you delivering pronouncements about the unsuitability of Python 3 when you don't have solid knowledge of how things are done in Python?
I did not "deliver pronouncements about the unsuitability of Python 3". I did only say, that it is not trivial to have both versions on one installation.
And who are you, something like the "Python purity police"?
I will ask you something, that you did not work with till now -- and when you give a wrong or incomplete answer, I will also say "how dare you"!
Actually, it's trivial nowadays with projects like pyenv. You can even have different python versions in a per-project-directory basis.
Check out the installation guide (https://github.com/yyuu/pyenv#installation) and a commands reference (https://github.com/yyuu/pyenv/blob/master/COMMANDS.md)
There's also an extension that provides virtualenv integration - https://github.com/yyuu/pyenv-virtualenv