I would not try and run two voicemail systems. IT just gets really messy. Plan a migration of all users to Unity, then move them to Callmanager. Cisco recommends this anyways. But in some cases, you have to run dual voicemail systems. But in the Octel, VPIM or AMIS world, it can be seemless. If you do not use this, then you will have limitations such as transfers, voicemail forwards, etc.
You also have to plan which auto attendent will be the primary. Having two AA, can screw with end user expierences when callig in. My vote:
Migrate everyone to Unity
If transfer do not exist in Unity, use the System transfer. (please enter the extension you wish to dial) Unity that grabs those digits, pushes it back out to the PBX for call processing. If you try to use subscriber system transfer and they are not in Unity, error will occur.
hope this helps