Is there no field that has a commonality between the data sources? Such as a date field or product name that could be used for the blending relationship.
Otherwise I believe this would require some sort of write back to store the totals and then adding the totals from the new location. Such as using the extension that you listed in your original post. There could be other options as well but I would to look into this a bit more.
I would also add an idea to the Tableau Community Idea space to have some sort of native grand totaling of non-associated data source totals.
Hope this helps!