Multiple forests with same local domain

Hello,

Before we drop this idea completely just wanted to check with the community if this setup is even possible. We have our main local domain which is configured with AD Connect and all devices are hybrid joined. This works well. Additionally, we have a multiple small separate domains, no trusts or connections between them and our main domain. User account and hostnames are unique and exist in single domain only. We wanted to use AD Connect to synchronize all of them into single Azure tenant and make all devices hybrid joined and enrolled into Intune. From what I understand, in this scenario you can only have one AD Connect server. This obstacle we can overcome. All separate domains have exactly same forest name in .local, let's say example.local. I know .local domain is not allowed, so we can create additional UPN to match our main office UPN. But is it possible to add multiple, separate forests with same UPN suffix into Connector?  Should I create unique UPN suffixes for each? A lot of users from those separate domain already have cloud only account because they use Exchange Online mailbox. Hope this is clear enough to give some answers.