I can easily bulk load INSERT into a junction object without issues.

However, I want to (UPSERT) with the Bulk API 2.0, into a many-to-many JUNCTION object (Custom object with 2 master detail records).

However, it appears that I need to use a externalIdFieldName that is WITHIN the JUNCTION object.  It doesn't sound like I can use the Master Detail record references.

What are some suggestions on how to upsert into a junction object using the Bulk Api 2.0?

Some thoughts or work-arounds that are not good are:
1. Create a new field on the JUNCTION object that is an External ID, and is simply a concatonated STRING from both of the "External ID"s from the other two objects
2. First download all the ID's for the Master records on both sides of the many-to-many Junction object, and then use those to "upsert" if needed.

Any help is apprecitaed, Salesforce documentation on the Bulk Api 2.0 is lacking and misses any notes about JUNCTION objects.