I do not believe you can use a result from a subreport to be a parameter (filter) for the main report.  A round about way would have two subreports where the main report is just a holding spot for the subreports. So you can put the first subreport in lets say the main report header and it will return (via a shared variable) a value, then in the main report footer you could have a second subreport that would use that shared variable as a parameter (filter) for the subreport.  I have never tried this, but I believe it would work.  It is kinda sloppy, but it might work.  Of course there may be a much better solution (help me out here!!).