Migrating multiple connections simultaneously to one database

We have multiple production applications (api.producta.com, api.productb.com, etc) that we want to migrate into a single database. When writing the user migration scripts, it appears that the database is expected to map to a single api via Login and GetUser scripts: Database ProductA points at api.producta.com.

Is there a good strategy for turning on a trickle migration from multiple sources into a single database? Or after finishing the migration for a product, can I at least migrate the users into another database? I want all the clients to be able to use a single Realm to login, not have to know what source product the user came from.

Is there any reply on this one available?

Encountering the exact same issue, what is the accepted solution to this?