I really commend you for looking for a responsible and kind solution to your LGD running off, because so many fatal or injurious things can happen to your dog when it's not home. As a shepherd who has has many LGD's over the past 40 years, I would highly recommend you not get a second dog. Your dog does not need another dog for companionship, he has his sheep. And you will have major complications adding another dog, I could go into it in detail, but suffice to say your sheep and your dog will be better off if you just keep him as a single guardian. Add the second dog when this guy's elderly and the new dog will be ready to take over guardian duties when Knox slows down.
I did mention that thought to my wife a few days ago. If he identifies with the flock as his companions, then that’s not an issue. My concern is not having a backup if something goes wrong or if he for some reason isn’t here. Seldom does adding an animal make things easier, so I am listening to you as I process this decision.