I have successfully accomplished this in an ad-hoc report that contained master data sets, but I had to create a MASTER master data set, if you will. I combined two master data sets into another master and created lookups to pull in the fields that were missing in either data set so the filter would work. The fields on the report must be present in both data sets for the filter to work correctly and the values must be the same in both data sets or it will return "various" in your report.
As an example, if the Asset Status field will be used on the report, but it's not present in both data sets, you must create a lookup to the other data set to populate the field. My lookup uses asset serial number to match the data in both data sets. It takes a little work to get it set up correctly, but it's worth it in the long run.
Hopefully that helps point you in the right direction.