Wendy what kind of report are you creating? Why would you need two rdls? I do know that if you have two datasets, you can create a detail line that contains both datasets as long as each dataset has a value that is shared between the two of them.

To give you an example, I created an open close report where I had a dataset of all of the projects and the number of defects opened. I created another dataset with all of the projects and the values that were closed. The final result was a single report where the detail line was a combination of those two datasets. Each column in the report, was a combination of those two datasets. Does that help any? I am somewhat unclear what the goal of your report is.