Anyone else having this problem? If either takes friendly fire from the other during the heat of battle, all hell breaks loose. Seems maybe a bit of a bug. The first few times was a nuisance, but this literally happens EVERY outdoor battle where both are present AND Serana will not follow (not even through doors to new areas) while they're busy slapping each other around. The only workaround, in fact, is to fast travel somewhere.

You're saying that when you're with Serana and Shadowmere, and a fight starts, if Serana accidentally hit Shadowmere they will start fighting eachothers?

LOL I find that funny.

Then just kill Shadowmere and forget it forever, Arvak is way cooler. And Serana is not replaceable.

This happens with anyone that serana accidentally hits, including guards, npc's, dogs, horses, etc.

It must be her vampirc period. Being undead, it never ends.

Shadowmere comes back though. Forgot how long it takes for him to respawn. Thing is, he respawns at the spot he died at, not next to you.

Circa 15-20 days later.

Hmm, are you sure that exiting the cell will not fix the problem? I took Lydia into Karthspire cave with Esbern and Delphine. Well, you know how Esbern just loves fireballs? So naturally, Lydia and him get into a fight. When Esbern went down on one knee, the fight would stop, but as soon as he got up again, the fight would restart. So I simply left the cave and Lydia ended up following. Once we went back into the cave everything was copasetic. Of course, Lyidia following me out of the cave may have happened during one of those periods where Esbern was down on one knee and not fighting.

Bah!
I beg to differ!
"Ardvark" will never bravely charge into battle at your side, crush pesky spiders and wolves that try to sneak up behind you, or cave in the head of that troll you never saw that was about to side-swipe you.
Shadowmere is irreplaceable!

Besides, Serana is essential. Beat her down for fighting the great Shadowmere until she takes a knee, you can't kill her. When she stands up again all will be well between them ....unless of course Shadowmere won't let it go. In that case I'd suggest dismounting as far from an expected fight as possible, and if it can't be helped then fight at Shadowmere's side and kill his target as fast as possible and remain between he and Serana. This way she'll be more likely to fight you than the horse.

I should know. Serana has attacked me more than a few times. She occasionally gets mixed up in the fray and will catch a wild swing from me. Completely on accident and sometimes unavoidable of course, and I can understand her irritation...but she should know better!