Messages with 2 iPhones?

My parents each have an iphone and they (finally) got their first mac last night. They share a primary iCloud account, but my dad uses his own for imessage on his phone. He uses nothing else with it.

Is it possible to get messages on Mountain Lion to use both iCloud accounts with both phone numbers or can I only get messages to work with one number at a time (currently my mom's cell).

Also, I know that they could each use an different account on the computer, but that's not an option. They have always shared an account. They work together, so they have all the data on a single account that they both use.