Does anyone know if it's possible to have 2 different Aras version installed (ie 11sp12 and 12.0) on the same machine for test purpose?
I assume il wil have separate databases and vaults.
This is most definitely possible. There are a few important things to note when installing your second instance. You'll have to ensure that the second instance has unique identifiers. This means it needs it's own file path, database name, conversion server path, and web alias. Each install will create a new database and each instance of innovator will have it's own vault. Going through the installation documentation in the 12 CD image should help you figure out how to set each one of these individually. It should be pretty straightforward, but let me know if you have any issues!
I made sure the second instance has unique identifiers but when trying to connect I get an error
"Error: Cannot access OAuth Server due to 500 (Internal Server Error)"
I checked the xml config files and don't see any mismatch with server names,etc.