Horses are all conformed differently like us which makes it difficult for a saddle fit to fit two horses.  I can assess the saddle for the two horses and can advise whether the one fit would be satisfactory - a main horse would need to be selected with a compromise for the second horse.  However it's really important that you understand that any changes in horse work, feed etc will impact the saddle fit and the two horses need to be assessed to ensure the fit is still appropriate.