The FileExplorer control is also handling the grid row dropping event (on the client) and checks if the target element is a tree node (drag&drop from the grid to the tree). If the element is not a tree node, the event is canceled. This means that when you try to drop a file explorer grid row in another grid, the event will always be canceled.
The only workaround I can think of now is to handle the grid row dropping client event yourself and check if the grid row was dropped in the other grid. If it was, do a manual postback or ajax request to execute your server code and update the controls.
the Telerik team