Suppose a form contains two edit controls linked to the CustomersTable and OrdersTable datasets through the CustomersSource and OrdersSource data sources respectively. When a user enters the edit control connected to CustomersSource, the navigator should also use CustomersSource, and when the user enters the edit control connected to OrdersSource, the navigator should switch to OrdersSource as well. You can code an OnEnter event handler for one of the edit controls, and then share that event with the other edit control. For example:

procedure TForm1.CustomerCompanyEnter(Sender :TObject);
begin
  if Sender = CustomerCompany then
    DBNavigatorAll.DataSource := CustomerCompany.DataSource
  else
    DBNavigatorAll.DataSource := OrderNum.DataSource;
end;

void __fastcall TForm1::CustomerCompanyEnter(TObject *Sender)
{
  if (Sender == (TObject *)CustomerCompany)
    DBNavigatorAll->DataSource = CustomerCompany->DataSource;
  else
    DBNavigatorAll->DataSource = OrderNum->DataSource;
}