You could end up finishing the game with both romanced without gaming the system, because the confrontation is linked to approval. And there are events and choices throughout the game that can lower companion approval without using the feastday pranks. Also Morrigan doesn't have to be near or at max approval to begin a relationship with the Warden, while she does have to have very high approval to trigger a confrontation over Leliana. That happened to me on my first time through the game and I had no idea they would confront you on it until visiting the forums.
In any case I would think the continuation of a relationship with Morrigan should entirely be linked to whether or not the Warden followed her through the Eluvian. Assuming you have a Witch Hunt Warden who was involved with both Morrigan and Leliana, I would think the Eluvian choice should terminate one of those relationships. If the Warden rekindles the old relationship with Morrigan and disappears with her, obviously Leliana shouldn't be waiting around for years for the Warden. She should move on. Likewise the decision not to follow Morrigan should end any romance path with her. Technically that romance path ended anyway at the Battle of Denerim, and Witch Hunt only provides an option to rekindle it.
As i said it is using game imperfection i don't get how peoples think that after you dump with morrigan or leliana (possible in brutal way) and then you give her mirror and expect she will say lets go have sex oh and can be with leliana im fine with it and then leliana says that same wha? Simple devs put flags on dialogues so if you take that no matter what you start romance even if you told leliana that you breaking up because morrigan is better in bed or say her that she is only girl that walks after you.Approval is only gameplay thing it doesn't exist in dragon age universe it shows your relations with companion but sadly it is imperfect system that is connected to the story.
So there is no
Leliana-oh you can sleep with morrigan im fine with that but don't get more than 70 approval because then i won't.
Simple every Li won't accept another they say you that outright and problem lies in that jealousy dialogue won't show second time simple is simple because game mechanic work that way.
Best example is when you romance zevran and didn't sleep with him wynne tells you that entire camp can't sleep because of you two or when leliana tells you that you are first person and last she see when she wake up and feel aslep (you need love approval to get this line but get it even if you didn't slept with her).