Table within a Table with different datasets

I have a report that has a table with another table within one of the textboxes. The table within contains a set of data items relating to the groups in the outside table. This was working fine until I split the dataset into two different datasets. When I change the contained tables dataset to a new one, I get the error that the report items expression can only refer to fields with the datascope. I tried to reselect the fields within the contained tables new dataset, but it only shows the available fields from the outside table dataset.