I may need some clarification on whether subreport2 is a child or subreport1. Regardless, one of these two solutions should work. 1. Put a Page Break element in the master report at the top of the detail band for the subreport2. 2. If #1 doesn't work, you can break subreport2 into its own report document (we'll call it MasterForSub2) and create a new report book and add Master Report to it and MasterForSub2 to it. They will definitely be on separate pages then.
