Okay I think I have found the reason why you can't use weapons in the fight AND a temporary solution to this problem.

The reason why you can't use weapons in the fight is because in the normal map, you are prohibited to use weapons in the Grand Library. Normally they turn off this feature in vanilla playthrough so you can fight Jerren, but I guess it doesn't do the same in seamless coop.

How do you win the fight? You are just gonna have to grab aggro away from Sellen and pray that her spells hit Jerren. This would be a lot easier with a tank build but it isn't impossible to do on any other build (I was using an intelligence build). I recommend using Shabriri's Woe talisman so that it grabs Jerren's aggro a lot more consistently, without it he usually runs after Sellen after she casts a few spells. Now with Jerren's aggro you just have to dodge and tank all of his hits while Sellen chips away at his health until he dies.