Well, not very seemless at all. When dealing with any networked Unity solution, you have to consider two things (and how exactly are they going to work between Unity systems):
1. Directory (Unity object information - subscribers, dls, etc...)
2. Messaging (the ability to retrieve and send messages)
There will be absolutely zero directory information sharing between each Unity server. There's no way to have the Exchange Unity recognize that a subscriber on the Notes Unity has been created, changed or deleted. There is no workaround for this.
There is the possbility for messaging. As long as the SMTP connectors are set up on the Exchange and Notes servers (i.e. you'd be able to send and retrieve emails from an Exchange user and a Notes user -- and vice-versa), you could have messaging between these servers.
Both Notes and Exchange Unity support what we call "Internet Subscribers". Think of it as a "contact" in Exchange. These subscribers do not have a mailstore, but they can have voice names, and DTMF IDs. By creating Internet subscribers on the Exchange Unity (these Internet subscribers would correspond to the Notes users on the Notes Unity), you could address, send and retrieve messages between Unity systems. If these Internet subscribers had the same voice names and DTMF Ids as the Notes users they represent, from the user's perspective, it's relatively "seem-less".
You would basically do the same thing on the Notes server; create Internet subscribers on the Notes server that correspond to the Exchange users on the Exchange Unity.
To be honest, it would be an administratively intensive (more honestly, a nightmare). Any changes on a Notes or Exchange subscribers would have to be manually detected and manually replicated to the corresponding Internet subscriber represented on the "remote" Unity. Once again, that's because there is absolutely zilch in the terms of directory sharing.
Here's a link to an excellent doc our pubs team whipped up on this subject matter....
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_installation_and_configuration_guide_chapter09186a008011a58f.html#1066836
There's other Unity networking docs out there. If you need help getting at them, let us know.
