I have the same question as Filter/search joined datasource dataview webpart which was asked about six months ago. This must be a common issue.
Since sharepoint basically combines two dataview's when using a joined datasource, I'm unable to filter based on the second dataview (the child). I can filter either rows of the parent or rows of the child. But the child cannot prevent the parent from not displaying particular rows, which I need it to when returning back search results.
My parent table is sql and the child is a Sharepoint list. My results have been that either it does not filter at all or I get an xslt error. Can someone help me?