The answer can be found Strange behavior of Windows Forms combobox control
Each combobox DataSource
property should be assigned to a different BindingSource
object.
Example:
cmbDataType1.DataSource = new BindingSource(datasource, "");
cmbDataType2.DataSource = new BindingSource(datasource, "");
Or in my particular case:
ddFrom.DataSource = new BindingSource(ctrlMap.GetAllCities(), "");
ddTo.DataSource = new BindingSource(ctrlMap.GetAllCities(), "");