Currently it is not possible to constrain a dropdown search field by another dropdown search field in a datagrid. Please add a feature request to our Partner Portal.
I'm not sure I understand your question. You want the Search Field of a datagrid to be constrained by the object selected in the datagrid? Seems like a strange use-case as having an object selected would negate the use of the search field, as you've already found a/your object... Also, if your search would activate when you select an object, the datagrid would reload and you'd lose your selected object etc.
What you could do is have a dataview that listens to your datagrid and have that view in turn contain its own datagrid.