Sorry, but I don't actually have a computer these days that has SSRS on it, such that I can test anything beyond my memory... As initially indicated, I would have normally accomplished this via a union query to create a single dataset that could readily respond to grouping levels (vs. as you've indicated, the resulting values from combining the 2 datasets only produces grand totals...)
Certainly wish that I could be of more help beyond the obvious suggestion to talk to whoever created the initial datasets and have them create a combined version for you to use as well? Sure hope that something works out for you. best wishes!