There is not a way to relate 2 tablixes.

An option is to use subreports. For more info, check out these sites:

http://technet.microsoft.com/en-us/library/ms160348(v=sql.100).aspx

http://www.youtube.com/watch?v=k_V4lam_5I0

http://technet.microsoft.com/en-us/library/ms160348(v=sql.100).aspx

On the subreport, create a parameter for the value that links the 2 reports. When you configure the subreport, pass in the field from the main report to the parameter on the subreport.

If you have any questions, let me know.

Sarah