Looking at your sample data, the easiest way for us to achieve the waterfall was to do a union of the two tables. You can do this by dragging in "New Union" from the Data Source page as you see in the below screen capture.
This helps us to append the two tables together, but since the "Project Sales Prev Year" table does not have a "Quarter" column, this will appear as "Null" in the new union table.
With this clean up, setting up the Waterfall will be easy to achieve, with resources like the following.
- Waterfall Charts | Tableau Software
In the attached workbook, you'll see the waterfall chart has "Null" for the previous year sales. You can edit the alias of "Quarter" to set Null as "Start" to make it relevant for your analysis.
Hope this helps!
Project Sample_k.twbx 24.1 KB