Light saber fights between two persons involve separating the fighters into two layers, having the light sabers on two layers and a bunch of other things like tracking. If you have shot your fighters on green screen then you can key some of the separations but you'll need to learn about rotoscope, rotobrush, motion tracking (not 3D camera tracking) and a bunch of other things. Being somewhat new to AE isn't necessarily a bad thing, but not figuring out that you need multiple layers to get the light sabers behind one actor is. An action sequence like that that is just shot without much planning and a hand held camera may take weeks or months to complete if your are an experienced user. Step one, learn the basics of rotoscoping and rotobrush and practice separating actors from the background effectively. Once you know the basics then start shooting some test footage so that you can shoot things that can be rotoscoped effectively. Well shot footage on a green screen will have just a few frames here and there where you'll need to separate an actor from the other actor so the fight will work without days of roto work by hand.
To start learning Rotoscope and rotobrush type Rotobrush in the Search Help field of AE and follow the tutorials. It's not intuitive at all so you must go through the training materials to have any success. Then practice on 3 or 4 second shots. When you get rotobrush down try researching rotoscope the same way. My friend Scott Squires has a very good explanation of the theory of rotoscoping on his site. Go through both parts of his instructions on his Effects Corner website: Effects Corner: Rotoscoping - Part 1
Here's a short tutorial I made on how to roto a subject by hand:
Thank you very much!