At first blush, this sounds like a UNION is what you need.
Are the columns in the 2 files identical? You indicate that you have an "Accounting Period" column, so I'm assuming the column structure is the same. If so, you can easily do a UNION of the 2 files (more or less, stack the second file on top of the first one) and Tableau will see it as one big table with all the accounting periods nicely lined up.
If that is NOT the case (e.g., your date periods in columns, so that the first data source has columns like 201701, 201702, 201703, etc. and the 2nd one has 201804, 201805, etc.), then you might have to PIVOT the data before you try a UNION (and you might not be able to UNION after the PIVOT).
The data is coming from two separate connections (queries) so, unfortunately, I do not think a union would work.