I'm not sure I fully understand your question.
However, you could create a local TDE that is a product of a cross database join. Then, publish that TDE to Tableau Online and utilize the Sync Client to refresh from the local source.
Is this what you had in mind?
1. Create CDBJ Locally
2. Create local extract file
3. Publish local TDE to Tableau Online and set Sync Client Refresh Schedule
If this helps or answers your question, please mark it as such so that others can find it quickly.
Tableau Online Deployment Advisor
Yes I think so! Thanks.