OK. So, you want the 'Header' from the Subreport to show up only when there is no header info in the Main Report right? Let's do it again.
1) I suppose you already have a Subreport in Group Header 1a? Make sure this Subreport only shows Header info. You may suppress all the other sections in this Subreport and just keep the section that shows the header info
2) Go to the Section Expert > Group Header 1a > Click the formula button beside Suppress and use this code:
Not(isnull({header_info}))
3) Use Group Header 1b to show Header info from the Main Report
4) Go to Section Expert again > Group Header 1b > formula for suppress:
isnull({header_info})
Replace {Header_info} field in both the codes with the database field for header info from your database.
Hope this makes sense.
-Abhilash