You can't directly access any control inside the gridview from code behind. You have to first make the object of that control.
GridView GridView2=(GridView)GridView1.Rows[RowIndex].FindControl("GridView2");
Something like this.
Refer this:http://www.codeproject.com/Articles/189997/Gridview-inside-Gridview-in-asp-net-Csharp. It may help you.