First off, personally, I would have put Hallie in the room, not Simba.

Like others have said, if they have been apart for a long time, then they will need time to adjust to each other. Keeping them separated for now would be best.

Okay, I just saw your post where you said they are doing better. That's good, but I would keep them separated for the night though.

How long will Hallie be with you?