The 100 migration batches limit and how to not run into it

This is a great feature, and have been testing it quite heavily.  It looks promising, however we've run into an issue that may be problematic.

User1 is in InitialBatch1

User2 is in InitialBatch2

We would like to move the 2 users out of InitialBatch1 and InitialBatch2, to a single batch InitialBatch3.

We are getting this error:

A unique batch must be used when copying users from one batch to another.

Have you come across this?

Thanks in advance!