Can I have multiple DB sources for the same dataset?

Short answer is YES, a dataset can use multiple data sources. One should simply do a full join that includes all records.

The UI only allows selecting existing fields to match them, so you can't use a condition like 1=1 Any help with that would be appreciated.

UPDATE: The matching rule doesn't really matter, FULL JOIN will always include all records from both sides.

UPDATE 2. Jan 2023 However, the result is a double set of columns, even if the two DBs and tables have identical column names."