So you're l33t at RPGs and have no idea how to start a fighting game, eh?
If you're actually serious about this, start by making a list of all the information you want to store about the player... position, move currently in, frame into the move, health, scancodes for each control key, etc... then set up your user-def'd type with all of that and DIM two of them for each player.
Plug in some dummy values, and load up some sprites out of one of your RPGs to use temporarily (drawing fighting game sprites takes forever). See if you can get it displaying the dummy sprites in the correct positions on the screen, and hook up the controls to move them back and forth.
If you get that working, work on some basic masking to keep the players on the screen, and to keep them from going on top of each other.
I liked my solution: Make a fighting game without the fighting part. Have you seen it? It's a projectile based duelling game. And there's a new d/l
here.