Supposing you will be using X and Y axis, I think you must define continuously the left and right limits (xL and xR) of your scene based on each fighter's position - let a margin of 1 or 2 meters behind each fighter, for instance - then alter the Z coordinate of the camera in order to get the borders at these points. Since the camera's view angle and the players plane form a triangle, the scene width will be proportional to the distance of the camera to the plane. You can set manually this distance at design time - let's call it zCam, and wScene the initial width of the scene. During the fight, you can set the distance of the camera to zCam*(xR-xL)/wScene, and the camera's X coordinate to the center of the scene, (xR+xL)/2.
Hope this algorithm works for you!

SCRIPTS ADDED: Create a new project; create the two fighters (simple capsules, for instance), name them Fighter1 and Fighter2 and set their Z and Y coordinates to 0. Add the script below to each one, and modify the Fighter2 command keys in the Inspector to "," and ".", for instance.

private var step:float = 5;
var leftKey = "z";
var rightKey = "x";

function Update(){

    if (Input.GetKey(leftKey)) 
       transform.position.x -= step*Time.deltaTime;
    if (Input.GetKey(rightKey)) 
       transform.position.x += step*Time.deltaTime;
}

Add the script below to the Main Camera:

var margin:float = 1.5; // space between screen border and nearest fighter

private var z0:float = 0; // coord z of the fighters plane
private var zCam:float; // camera distance to the fighters plane
private var wScene:float; // scene width
private var f1:Transform; // fighter1 transform
private var f2:Transform; // fighter2 transform
private var xL:float; // left screen X coordinate
private var xR:float; // right screen X coordinate

function calcScreen(p1:Transform, p2:Transform){
    // Calculates the xL and xR screen coordinates 
    if (p1.position.x<p2.position.x){
       xL = p1.position.x-margin;
       xR = p2.position.x+margin;
    } else {
       xL = p2.position.x-margin;
       xR = p1.position.x+margin;
    }
}

function Start(){
    // find references to the fighters
    f1 = GameObject.Find("Fighter1").transform;    
    f2 = GameObject.Find("Fighter2").transform;
    // initializes scene size and camera distance
    calcScreen(f1,f2);
    wScene = xR-xL;
    zCam = transform.position.z-z0;
}

function Update(){

    calcScreen(f1,f2);
    var width:float = xR-xL;
    if (width>wScene){ // if fighters too far adjust camera distance
       transform.position.z = zCam*width/wScene+z0;
    }
    // centers the camera
    transform.position.x = (xR+xL)/2;
}

This script follows the fighters. If they fit in the scene, the camera just centers itself. It they get too far, the camera zooms out in order to catch the entire scene.