I doubt they'll ever get on, you could separate them but IMO its less hassle to part with one. The older one might settle down with plenty of graft but I think you'll always be keeping an eye on them. I had a similar problem but it was the pup finding his feet started on the older dog, he was alot bigger & a right hand full but was turning into a promising dog so I sent the older dog to a keeper mate who still has him. The younger dog served me well but was always a b*****d, he mauled a labrador & was PTS last year