I think this is possible, though I've not tried it. The reason why you have to link a report to its parent in CRM is so that CRM can update the path in the parent report to the sub report.
So, you could try setting the sub report's paretn to one of the parent reports, then changing it to a different parent report, and the path in the original report may stay correct
Microsoft CRM MVP - http://mscrmuk.blogspot.com/ http://www.excitation.co.uk