Salesforce Batch Create question. Creating Junction Records?

I’ve been experimenting using the SFDC Batch Create snap to create a record and associated child records in a single transaction, and that works fine, as advertised.

What I can’t figure out how to to is to do the following: Insert a record in each of two (unrelated) SObjects and create a Junction record in a third SObject, that connects them both, all in a single call. And fair enough, the Snap Pack documentation doesn’t suggest I should be able to do that.

But I know that under the hood this Snap is using the SFDC REST API composite Resource, and I know it supports this kind of thing. Example Here

I tried various ways to do something similar in my JSON source for the Snap, but no matter what I try it just bombs out with an error.

Should I be able to do this? Just curious.