So if I understand you correctly, this is what you want to do...

DataGroup1 Header
  DataGroup2 Header
     Detail
     Detail
  DataGroup2 Header
     Detail
     Detail
DataGroup1 Footer <--- You want this on a new page!!

Question ... Do you want anything else on the DataGroup1 Footer page ... or should it be all by itself? If you want it all by itself, try defining the height of the datagroup1 footer to be the max height of the page, less any margins, page header, and page footer bands. That should force it to start on the next page.