Not sure why its giving you a hard time. I had built a dashboard that compares the current year vs the past year two different datasets. Here is from one of my cells
=CountDistinct(Fields!HCPC_Enabling_pt.Value, "dsMain")/CountDistinct(Fields!HCPC_Enabling_pt.Value, "dsPast") for some reason I have no problem with using two datasets.
***SQL born on date Spring 2013:-)