The short answer is yes! but it does come with a few words of caution. Managing two databases/companies from the same database can seem like a good idea at the start but it's something that you would need to manage closely to avoid communicating with the wrong people sending the wrong message and getting a lot of unsubscribes, etc.
Within the account, there is one contact list, however, using groups and/or custom fields you can help segment these contacts to start building your database.
If you succeed with a good, clean split, you also have to be mindful that you only have one suppression list, this is where all of your unsubscribes, complaints and hard bounces end up (for more information on the suppression list please click here). This means that if someone on your database unsubscribes from one company, they unsubscribe from any and all companies that you run off your account.
So it is possible but it is something that you need to be careful with