Multiple tenant one domain

Hi, we have a customer that has Exchange 2016 On-premises, domain newco.net. It wants to add 2 different companies, test1, and test2, that have their own mail domain (test1.net and test2.net).

The goal is to unify the mail domain into newco.net, migrating to Exchange Hybrid. Test1 and test2 mail accounts will be hosted on the office365 tenant. Is it possible with power365 email rewrite services to create Tenant A (to manage accounts from test1)and Tenant B(to manage accounts from test2) that manage newco.net as a mail domain?

So will have users that will remain on exchange On-premises, users from test1 that will be managed by tenant A, users from test2 managed by tenant2, one domain mail, newco.net, one Active Directory that will host all mail account .

Is it possible?

Best Regards