I am having trouble using the RollUpList type with multiple lists from the same site in one dataset.  We want to join data from 2 lists within a site collection into our report.  If I attempt to use 2 MultiList definitions using the RollUpLost type, I recieve the following error:

"A DataTable named '[relativeURLof subsite]' already belongs to this dataset."

I can get this to work if we use the MergingList type but not the RollUpList type.  Is there a workaround for this scenario?

Thanks.