I'm pretty new using Moodle, but I have to administrate one, and the question is that if I can have two versions of moodle on the same server, because I have a old stable version installed but I want to make some tests with the newer version and have both on the same server, this is possible?..
Yes, you will need to create separate directories for the code and the data and a separate database for your Moodle tables. When you do the second Moodle install, you will be prompted to input the path to your data directory and the name and login info for your database.
Should you have a limited number of databases available on your sever, then it is possible to use a different prefix for each moodle installation, using the same database. The prefix can be in set in config.php.