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"!