How could Drupal 6 and 7 live together in the same multisite environment?

I imagine this subject has been discussed endlessly, but unfortunately not found any solution yet.
I have a multisite environment in Drupal 6 working perfectly, but I want to start new sites now using Drupal 7, without converting the previous sites.
How would it be possible to have versions 6 and 7 in the same domain?