Not sure,but you may try Tableau Prep for this.
The ability to join JSON files is not currently built into Tableau Desktop. There is an existing Idea in the Community which you can vote up:
JSON files from two different sources and would like to JOIN them.
The current workaround would be to connect to each file separately in Tableau Desktop, export the data as a csv or to Excel, and then connect in Tableau Desktop to the csv or Excel files, where they can be joined. Or, try blending.
Now that we can join JSON files can we open this question back up?
As far as I can tell by testing Tableau Desktop 2019.3, the ability to join JSON files with other JSON files is not built into Tableau Desktop. JSON files can be unioned but not joined.