I would create a Linked Server in MS SQL Server pointing to Oracle. Then you can union results from local query and openquery from linked server.
You could also create a calendar table and use that as you primary source in Tableau joined with the two other data sources on dates.
I do not have access to the database except for read permission .
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You cannot use custom SQL across two different datasources and you cannot union the data with blending in Tableau.
You need to pre-process the data before you load it into Tableau. Either manually pull it together in Excel or look into something more automated like linked the servers as suggested above or pulling the data into a data warehouse.
You may take advantage of Linked Server even in the free SQL Server Express version.
I have attached a sample workbook with data blended from 2 salary tables. As I understand the problem is that some dates may be missing from one or the other source and therefore inability to fully blend that data. The solution is using calendar table with full range of dates we want as primary source in Tableau. That is because blending works similar to LEFT JOIN and your primary source is to the left.