I know this may sound harsh, but why not try an invisable fence? Do not buy the ones inb the store, the wire is too thin & will break with 1-3 yrs. The private invisible fence r good ones, the wire is much thicker & lasts much much longer. Also get Brock the heavey duty one.

The fence may act as a correction aswell. Everytime Brock wants to dig or attact the other dog, he will be warned & then zapped.

It just might work.