Map 'em out with action figures. It's dorky as all fuck, but it really helps keeping who's doing what straight in your head.

For writing the moves, watch more media that features the sort of fighting you want to write, and pay attention to the flow of the scenes. You don't have to be overly technical. As long as you convey to your partner where your character's swinging and with what, or where they're shooting at, you can focus more on feeling and thought process than making sure you have the technical details nailed down.