Do any companion quests interfere with each other?
Solution 1:
Interference, yes. Increasing difficulty to complete? Maybe, depends on your decisions. Making it impossible? No.
Sebilles and Red prince have some crossing points in their quests. They involve the same NPCs but for different reasons. Your decisions may make one of the quests more difficult or even impossible, but with the right choices you are perfectly able to complete both.
This is the only interference, so if you want to be safe, pick only one of the two for your party.
Solution 2:
Since I've started playing the DOS2 again, let me be more precise:
There are two situation in game where Sebille's and Red Prince's quests interfere:
In Fort Joy with the lizard called Stingtail, who is involved in three quests:
- Red Prince wants to consult him regarding his future
- The cook Griff will want to kill him (if you point that he is the thief during the "Imprisoned Elf" quest)
- Sebille wants to kill him, since it was the Stingtail who captured her.
You can complete all three quests if you do them exactly in this order:
First (without Sebille in your party) let Red Prince to have his dreaming session.
Then (still without Sebille!) get stolen goods from Stingtail and leave most of your group near him. Get back to the Griff, return the goods and point to Stingtail as the thief (otherwise you'll have to fight big chunk of the camp and you won't be able to progress Sebille's quest). Griff will send assassin that will spawn near Stingtail and start attacking him - join the fight with the team members that you left there and help the lizard.
FINALLY get Sebille and approach the poor Stingtail. After brief "conversation" she will kill him.
The second time it occurs on the Nameless Isle with character called "Shadow Prince": as previously, Red Prince will want to talk to him while Sebille wants to kill him. Simply move Sebille away from the group (or even remove her), let Prince do the talking, then get the crazy elf into your group and let her get the revenge on her tormentor.