Multiple Azure Active Directories against a single Forest

I have seen this question popping up and wanted to share the guidance that was recently highlighted. This could occur where you have two organizations in one forest and want to separate that organization from Office 365. Note, from the guidance, only one hybrid exchange environment can be supported, the rest of the organizations will need to be O365 only.

https://azure.microsoft.com/en-us/documentation/articles/active-directory-aadconnect-topologies/\#multiple-azure-ad-directories