If you were to create a DAG and add another Exchange Server, then replicate those 2 databases to the new server, then each server would have 2 databases each.
That would allow you to add 3 more databases and replicate them to each server - 5 total on each.
Once they reach 1TB, you either add another DB and move mailboxes ( as long as each server has 5 or less DBs) - or - upgrade to Enterprise - then you dont care - add as many as you need DB-wise.