I'm using PyGObject but I think this is a question that could be adapted to all GTK, so if someone know how to do it using C or anything should work in python also.
I have two treeview, Active and Inactive, I load data from a Sqlite database and I can swap and drag & drop items from one to other. 
This is just an aestetic thing, if I click on one item on one treeview I want that a previous selected item on the other be deselected.
It appears that nobody had to do something similar because I didn't found anything about it on the net.
At the risk of being too basic (perhaps I misunderstand the problem), to manipulate treeview selections, you use the GtkTreeSelection object returned from GtkTreeView.get_selection.  You can attach to signals on this object, change the current selection,etc.
Gtk.Treeview deselect row via signals and code