217 votesMatthew McKee commented
This is also something my company deals with. We have 2 separate domains for marketing two lines of business, each with their down office 365 instance. Some people only exist in one, while others exist in both, but because they are separate entities, they can't be the same AD users with aliases/multiple inboxes. This leads to have separate SfB accounts and while one instance of outlook can manage both emails, I can't do the samr with Skype for IM profiles.