In 2018.2 Tableau simplified the process to accomplish this
see the link and view the Spatial Join feature
If this posts assists in resolving the question, please mark it helpful or as the 'correct answer' if it resolves the question. This will help other users find the same answer/resolution. Thank you.
To create the dual axis map, it looks like you just need to join the two data sources together, instead of using them as separate data sources. I suggest joining on a calculated field (0 = 1) with a full outer join so that you end up with a single data source with all rows from both tables. There are some instructions here: Dual Axis Mapping - Many Ways See the section on spatial file + csv.
I cannot access the source for your Google sheet data to make an example, but there are a few example workbooks attached to the post that I linked above that might be helpful in solving your problem.
If you need to be able to identify which point falls inside each polygon, as Jim noted in his earlier response, you will want to try the spatial join functionality in Tableau 2018.2 or later, though to use this you will have to convert your Google sheet with latitude and longitude values into a spatial file type (e.g., shapefile, KML, etc.).
Thank you so much Sarah, that indeed solved my problem.