Perhaps pass the existing DataSource to your new form? Note that the
designer doesn't help you do this - but there is absolutely nothing to
stop you from creating something like below... then just update
DataSource [and possibly DataMember] when creating the sub-form.

public SubForm : Form {
       // bs created / disposed as normal; combos bound to this
       private BindingSource bs;

       public object DataSource {
           get { return bs.DataSource; }
           set { bs.DataSource = value; }
       }
       public string DataMember {
           get { return bs.DataMember; }
           set { bs.DataMember = value;}
       }
}