Can you do just one instance of 2008 and run the 2005 database in compatiliby mode? That only means 1 license.
Let me just rephrase it.
'You want to configure ONLY 1 instance on 1 physical box (no VMs at all) and multiple databases (per application, different compatibility modes) with 1 license (assuming valid processor license)’.
Yes, it’s a valid licensing per my understanding. But you must crosscheck with Microsoft team once.