You're getting this error because DivisionID is being returned twice, once for each file. Usually when you are setting up a multiple table join in Tableau, Tableau will assign one result to DivisionID and rename the other to something like [Divisions#txt].[DivisionID] AS [Divisions#txt_DivisionID] to avoid this problem. If you manually edited the fields to return DivisionID in both cases, then change it to something else. If you did not manually edit the fields, there's something strange going on in Tableau and I'd recommend contacting Tableau tech support.
I'm afraid I'm in the strange zone - I did not manually edit the fields.
When I did it with excel file - it went well