Hi Cerzar,
Based on your description, you have multiple forests on-premise environment. And you want to integrate both organizations with a single Office 365 tenant, deploy single sign-on (SSO), and perform migration.
I am afraid we cannot meet this requirement currently. It is not feasible to integrate two organizations with Office 365, and DirSync is also not available in this environment.
Moreover, I suggest you submit feedback to our product team. If many customers have the same environment with you, and you are concerned about this feature, it may be designed better in the future. As far as I know, many features are improved based on customers’ feedback. I appreciate your effort and contribution.
Thanks,
Jason Jiang