I would be guessing that you would be looking at trouble. There is a very good chance you could lose one to the other. If you had a large tank you may be ok for a little while. Depending on the type of eel. But mantis can be very aggressive and will defend there territory with a passion.
IMO It would probably be good practice to keep a mantis in a species only tank. That way you can add a cheap fish like a damsel for a little extra movement and color. If the mantis kills it you would not be out a great deal of cash and they are easily replacable.
On Discovery ,I seen a large mantis defend itself VERY well against an octopus. From the octopus I had a couple of years ago that is a very big feat. When they get something into there head that they are going to eat, they are a very determined creature. They are very smart and cunning, and that mantis had no problem changing the octopi's mind.
Like i said IMO mantis Species only to be on the safe side.