Which Sections are you utilizing for each report? Which section does the sub-report print?

It might be simply a matter of inserting an additional Page Header section (PageHeaderB) onto the Main Report that does not contain any fields. Set the height of the PageHeaderB section equal to the top half of the report. Suppress it if the Page Number = 1 and suppress PageHeaderA if the Page Number > 1.

There are other methods but again they would depend on which sections you are using.