Can my client synchronize two stand alone forests with a single office 365 tenant?

I have a client that has two internal active directory domains in separate forests.  Let's call them domain_A and domain_B. The domains use separate UPNs, blue.org and red.com.  A separate Exchange server is installed in each forest.  User accounts are not synchronized between domains by ADFS or any other means.  (300 Users in Domain_A and 100 Users in Domain_B_

We are successfully synchronizing domain_A to Office 365 now.  Is installing a separate ADFS and DirSync server in Domain_B and syncing those objects supported by MS?  What are my options?

I believe this is not supported but am looking for one final confirmation to share with my client.

At this point, I'm leaning towards using manually created and managed "In Cloud" accounts for all Domain_B users - OR- moving all accounts to Domain_A, establishing a Domain Trust, and using Domain_B as a resource domain.  The changes that will be needed to the PC Profiles is the only hard part (100 PCs / Users).

Suggestions Please?

Thanks