IMHO, the best way to use two different Python versions on macOS is via homebrew. After installing homebrew on macOS, run the commands below on your terminal.
brew install python@2
brew install python
Now you can run Python 2.7 by invoking python2 or Python 3 by invoking python3. In addition to this, you can use virtualenv or pyenv to manage different versions of python environments.
I have never personally used miniconda but from the documentation, it looks like it is similar to using pip and virtualenv in combination.