Use "python3 -m venv"
You can activate two different environments in two different terminals. Why wouldn't you be able to?
You can automatically activate venvs when you cd into a directory with virtualenvwrapper.
Rule of thumb: if a thread goes into C it's releasing the GIL. Not always, but often. Like reading a file, or a library like like numpy or Pillow.
Then there is the GIL. I still don't know if multithreaded python apps are really MT or just faking it. And yes, I know you can do event driven stuff and that does look very interesting.