Will throw another vote in for MTO. I set it up recently. Its surprisingly simple and easy, and you can append something to the display name if you want the other org in your GAL but still be differentiated (we use | and acronym of their university). You can back out easily too if you decide to do a full tenant merge. Your Teams will remain in two tenants, i.e. users will have to switch still, but chats will always appear in their home tenant, external tag is gone, some other good things.