If the data source is the same database, I can think of 2 potential ideas if the data sources are really one sql db.
1) create a custom sql connection in Tableau using a Union Join between the two tables with the same exact field names. This could be treated as a single connection and incrementally updated.
2) on the server side, create a view that contains the combination of both tables and connect to this and incrementally refresh.
If the data sources are entirely separate though, and the goal is to create essentially a union join between two disparate data sources, then I can only think of solutions outside of Tableau (ETL) to do this sort of thing.