If you only need to pick one attribute from each field maybe using parameters created from fields would work for you.
Or maybe creating a combined field and using that as the filter.
I don't see how creating a parameter will help.
I wrote that I tried creating calculated measure (combining dimensions from the data sources) to use as a filter, but then I can't aggregate on my table properly.
It would look something like this:
IF ATTR(D1.a)="x" and ATTR(D2.d)="y" then "Filter Me"
Then if I add the filter, the "Filter Me" option will not show up unless I add certain dimensions to my display - which I do not want to do.