Office 365 \ Exchange 2010 Hybrid upgrade

Johan Pauly  - hi Johan. What are the mx records pointed to for each domain? (onprem or o365?)

How many users need to be migrated to domain2.com?  Also can you confirm you are using 2 separate o365 tenants or is this a single tenant with multiple domains?

I believe if it is a low number, the best method would be to create empty mailboxes for them in o365, cutover the mx records, and wait for mailflow to go to their o365 accounts (TTL 1hr?)

Then  simply export the mailboxes to pst and perform a network upload to their corresponding mailboxes manually. This may not be viable if your user count is in the thousands, but for a small to midsize org it should avoid the complexities of connecting your onprem exchange organization to more than one o365 tenant.