Hello - I didn't havethis issue personally but here's what I would do. Ease them out gradually. First, like Susan said, put their mattress on the floor in your room. Be firm and be the parent - kids are smart and will play on your weaknesses. Do that for say 1 week. Then start moving the mattress further away. (Not sure how far away your 2 bedrooms are though). When their mattress makes it to their own room you can buy them new sheets, new PJ's, a lamp, special stuffed toy etc.
I do remember my son having an issue with "Monsters under the bed" for a while and I had a magic protection spray that only Mommies can buy. We would spray his room down every night and tell the monsters that they cannot come in. I just made up a cool can cover on the computer and tape/glued it on to a can of air freshner. I only ever used that scent in his room though!
Just be consistant with what you decide to do. They do grow out of it. Also check out a book or movie that helps with night fears.
Good luck
M. F