Here's something that I've done. I have a few Crystal reports that have subreports in them. I was trying to created an SSRS report exactly the same as the Crystal report. Well it wasnt coming out exactly as I wanted it.
I solved my issue with one report, without using any subreports in SSRS.
If both reports are using the same query, great; if not, you can always add another dataset. Here goes:
1)Insert a List object.
2)Go to the tablix reporties and group it. Choose a field that works for you.
3)Insert a Rectangle object into the List object. (This object will fill in the tablix cell. So will look like there's no object in the tablix, but there is.)
4)Create your main tablix that contains your "MainReport" and then create a new tablix that's your "SubReport."
5)After you're done creating both tablix, insert them into the Rectangle object.
6) once your insert both tablix into the Rectangle, you can pretty much free form within the Rectangle object.
You probably have to mess with the Page Break options to have it display correctly, but this worked for me for several reports. So instead of working with two reports, one thats your Main Report and the second which is the SubReport, you only have one report to deal with.