the way you have it setup, I don't think you will be able to foward messages between the system. You have to remember that Unity rides on the back of AD and Exchange. Think of forwarding an Email message from Exchange server to another. The only way this could happen, would with SMTP. The way Unity does this between other Unity Servers, is to use Digital Networking. The way digital networking is used is basically the same as multiple Exchange servers in the same Org or AD infrastructure.
If Unity UM and Unity VMO were connected in the same Domain, only one Exchange server could exsists. Therefore, only one AD. You can't have two seperate Exchange servers in a domain. They have to belong to the same Exchange Org. If you had Two Unity UM servers, they would digtially networked through AD/Exchange and Unity Networking. Voicemail users could foward messages to whomever is on the system.
Sounds like you are also managing two seperate directories also. Therefore, if you were to foward a message, by name or extension, the Unity server is not aware of the other Unity server.
You best bet, if not mistaken here... you would want to migrate the VMO users to the UM unity server. You can create accounts such as "John Doe-VM" so you can clearly see in AD, who are only VMO accounts and who are UM users, etc.
I'm not sure a Unity Bridge would work... it's not really used for this application, more of an Octel bridge scenario.
