Two lasers between two mirrors

JavaScript (ES6), 149 bytes

Takes input in currying syntax (w)(h)([a,b]).

w=>h=>g=(p,d=[1,-1],s=Array(w).fill` `)=>h--?`|${p=p.map((x,i)=>~(k=d[i],s[x]='/X\\'[x-p[i^1]?k+1:1],x+=k)&&x<w?x:x+(d[i]=-k)),s.join``}|
`+g(p,d):''

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Commented

w => h =>                  // w = width, h = height
  g = (                    // g = recursive function taking:
    p,                     //   p[] = array holding the point coordinates
    d = [1, -1],           //   d[] = directions
    s = Array(w).fill` `   //   s = array of w spaces (we can't use a string because it's
  ) =>                     //       immutable in JS)
    h-- ?                  // if we haven't reached the last row yet:
      `|${                 //   append the left pipe
      p = p.map((x, i) =>  //   for each x at position i in p[]:
        ~(k = d[i],        //     k = direction for this point
          s[x] = '/X\\'[   //     insert either '/', 'X' or '\' at position x in s
            x - p[i ^ 1] ? //     if p[0] != p[1]:
              k + 1        //       use the direction
            :              //     else:
              1            //       force 'X'
          ], x += k        //     add k to x
        ) &&               //     if the result is not equal to -1
        x < w ?            //     and is less than w:
          x                //       use the current value of x
        :                  //     else:
          x + (d[i] = -k)  //       change the direction and restore the initial value of x
      ),                   //   end of map()
      s.join``}|\n` +      //   join and append s; append the right bar and a linefeed
      g(p, d)              //   followed by the result of a recursive call
    :                      // else:
      ''                   //   stop recursion

Stax, 40 bytes

àù@○⌡┼PY¼îαφu^·A☺°É⌠■╟¡Åt^◘v(µ╩Ñ♣t{╓○xß╦

Run and debug it

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Pretty sure this can be further golfed.

Input is given in the form of width [right-going left-going] length(per comment by @EngineerToast).

ASCII equivalent:

xHXdmzx);hi+x%Y92&;Hi-x%cy=41*47+&2ME:R\{|+m'||S

Python 2, 119 bytes

w,l,a,b=input()
exec"print'|%s|'%''.join(' \/X'[sum(i==k%(2*w)for k in[a,~b]+[~a,b]*2)]for i in range(w));a+=1;b-=1;"*l

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