Matrix w/Group Report with sub reports

 I have a relatively complex report to write using a matrix report as my base, then adding two additional nested reports to the body of the matrix.  When I add the second SQL statement, it all looks as though it is going well...but as soon as I try to nest a report within the first report I get an error telling me that a cross product can only have one child group.  What is a cross product?  Is this referring to my matrix report?  If so, what qualifies as a child group within that product?