This is a separated list of 2d and then 3d game engines, sorted alphabetically.
Flixel is simply the easiest game engine to use in an ActionScript 3.0 project. It has basic movement physics, including pushable objects, inertia, and gravity. It renders a single bitmap as its screen output and could work well for a split-screen application.
Unity Pro is a commercial alternative to Maratis and quite possibly a better option. There is a free edition of Unity with watered down features. Unity is extremely impressive, but the cost of the engine alone may scare you away.