Rowan, I have a mod installed that lets me have more than one companion. I don't like to over-tweak my game, but this mod gave me a few options I really wanted: my companion can ride her own horse, she ignores friendly fire, and I can have another follower if I want. Right now, I have Lydia for a follower and two horses. She rides the second horse. And of course, Barbas is tagging along. I guess the advantage of Barbas is that he can't die. I think I'll postpone finishing his quest and see how we get along.
Thanks for the suggestion! Amazing Follower Tweaks