dual license

Please forgive me my ignorance, but this licensing stuff is quite difficult :). I have two simple questions:

1. Is it allowed to distribute a Drupal module having a dual MIT+GPL license?
2. Is this module allowed to be put on Drupal.org as an official Drupal module?

So basically my problem is understanding the relationship between MIT and GPL. They are compatible, but what does that exactly mean? (p.s. I read sites like http://stackoverflow.com/questions/3336161/mit-gpl-dual-license-in-comme...)