virtualenv with python2 and python3 via Homebrew
According to this SO question, you could use a single virtualenv for both version, just use the -p path/to/python/version or --python=path/to/version flag to specify which version to use for the creation of the virtual environment. Note that you can use the same flag with mkvirtualenv.
This post recommends to just use the system Python (2.7 in your case) to install virtualenv itself.