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

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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Python 2, 168 bytes

w,n,r,l=input()
R=L=1
exec"""
d=[~r*R,-~l*L].count
print'|%s|'%''.join(' /\X'[2*d(~x)|d(x+1)]for x in range(w))
if-1<r+R<w:r+=R
else:R=-R
if-1<l-L<w:l-=L
else:L=-L"""*n

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Python 2, 187 181 179 177 174 172 171 bytes

def f(w,l,a,b,A=1,B=-1):
 while l:l-=1;print'|%s|'%''.join(' \X/'[[0,A,B,2][(i==a)+2*(i==b)]]for i in range(w));a,A=[a,a+A,-A,A][-1<a+A<w::2];b,B=[b,b+B,-B,B][-1<b+B<w::2]

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Recursive:

Python 2, 172 bytes

def f(w,l,a,b,A=1,B=-1):
 if not-1<a<w:A=-A;a+=A
 if not-1<b<w:B=-B;b+=B
 if l:print'|%s|'%''.join(' \X/'[[0,A,B,2][(i==a)+2*(i==b)]]for i in range(w));f(w,l-1,a+A,b+B,A,B)

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Recursive, alternative print:

Python 2, 172 bytes

def f(w,l,a,b,A=1,B=-1):
 if not-1<a<w:A=-A;a+=A
 if not-1<b<w:B=-B;b+=B
 if l:L=[' ']*w;L[a]=' \/'[A];L[b]=[' \/'[B],'X'][a==b];print'|%s|'%''.join(L);f(w,l-1,a+A,b+B,A,B)

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C (clang), 240 236 208 bytes

#define g(a,b) b?a++,a==x&&(a=x-1,b=0):a--,a==-1&&(a=0,b=1)
i,m,n,o,p,t[]={47,92};f(x,y,r,l){for(m=1,n=0;y--;puts("|"),g(r,m),g(l,n))for(printf("|"),i=0;i<x;o=i==r,p=i++==l,putchar(o*p?88:o?t[m]:p?t[n]:32));}

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f() takes parameters as follows:

x = width,
y = length,
r = Initially right-going line starting position
l = Initially left-going-line starting position

-4 Bytes. credits Kevin Cruijssen. Thanks

Canvas, 66 40 bytes

｛²Ｘø⁶╵［⁷／²２％［↔｝∔｝；ｘ╷？⁷∔⁷＋╷｝［���｝｝ｎ⁶［|ＰＪｐ|ｐ

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Charcoal, 56 50 bytes

↷ＰＩθＭ⊕η→ＩθＦ²«Ｊ⊕⎇ιεζ⁰ＦＩθ«✳§⟦↘↙⟧ι∨⁼ＫＫψX¿⁼ＫＫ|«¿ι→←≦¬ι

Try it online! Link is to verbose version of code. Edit: Saved 6 bytes by reducing reliance on pivoting. Explanation:

↷ＰＩθＭ⊕η→Ｉθ

Print the sides.

Ｆ²«

Loop over the two lasers.

Ｊ⊕⎇ιεζ⁰

Move to the start of the laser.

ＦＩθ«

Loop over the height.

✳§⟦↘↙⟧ι∨⁼ＫＫψX

Draw a \ or / in the appropriate direction, unless the square is not empty, in which case draw an X.

¿⁼ＫＫ|«

Have we hit a side?

¿ι→←≦¬ι

If so then take one step sideways and invert the direction of travel.

Java (JDK 10), 186 bytes

(w,h,r,l)->{var x="";for(int i=0,j,R=1,L=-1;i++<h;l+=L,l+=l<0|l>=w?L=-L:0,r+=R,r+=r<0|r>=w?R=-R:0,x+="|\n")for(j=0,x+="|";j<w;j++)x+="/X\\ ".charAt(j==r?j==l?1:R+1:j==l?L+1:3);return x;}

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PHP, 177 169 166 bytes

[,$w,$h,$a,$b]=$argv;for($e=-$d=1;$h--;$s[$a+=$d]^L?:$a+=$d=-$d,$s[$b+=$e]^L?:$b+=$e=-$e){$s=str_pad("",$w)."|";$s[$b]="X\/"[$e];$s[$a]="X\/"[$a-$b?$d:0];echo"|$s
";}

requires PHP 7.1 for negative string indexes, PHP 5.5 or later for indexing string literals.
for PHP <7.1, remove ^L, replace "X\/" with "/X\\", :0 with +1:1, [$e] with [$e+1], remove ."|" and insert | before the newline. (+3 bytes)
for PHP < 5.5, replace "/X\\" with $p and insert $p="/X\\"; at the beginning. (+2 bytes)

takes input from command line arguments. Run with -nr or try them online.

Python 3, 162 bytes

from numpy import*
def f(w,h,u,v):
 v=w+w-v-1;T=eye(w);M=vstack([T,2*T[::-1]]*2*h)
 for r in M[u:u+h,:]+M[v:v+h,:]:print('|%s|'%''.join(' \/X'[int(i)]for i in r))

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Ruby, 117 bytes

->w,h,a{a[1]-=w;(1..h).map{s=' '*w;a.map!{|x|d=x<0?-1:1;s[x]='X\\/'[s[x][/ /]?d:0];x+=d;x==w ?-1:x<-w ?0:x};?|+s+?|}}

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Anonymous lambda taking input as width w, height h and an array of starting points a.

PowerShell, 243 233 222 205 bytes

param($w,$h,$a,$b)$l,$r,$s=1,-1,' \/'
1..$h|%{$p,$p[$b],$p[$a]=[char[]](' '*$w),$s[$r],($s[$l],"x")[!($a-$b)]
if($a+$l-in($z=0..($w-1))){$a+=$l}else{$l*=-1}if($b+$r-in$z){$b+=$r}else{$r*=-1}"|$(-join$p)|"}

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Oooof. those logic blocks are big and dirty and mostly duplicated. The next step would be rewriting them so they don't need the else statement.

Python 2, 165 164 bytes

w,h,x,y=input()
a,b,s=1,-1,' \/'
exec"""l=[' ']*w
l[x],l[y]=s[a],s[b]if x-y else'X'
if-1<x+a<w:x+=a
else:a=-a
if-1<y+b<w:y+=b
else:b=-b
print'|%s|'%''.join(l)
"""*h

Saved a byte thanks to Jonathan Frech.
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K (ngn/k), 58 bytes

{" \\/X|"4,'(+/3!1 2*(x#'e+2*|e:=2*x)(2*x)!z+(!y;-!y)),'4}

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anonymous function that accepts three arguments: x the width, y the length, z a pair of starting positions for the lasers

C (gcc), 169 bytes

A,B,c;f(w,l,a,b){for(A=1,B=-1;l--;a+=A,a<0|a==w?A=-A,a+=A:0,b+=B,b<0|b==w?B=-B,b+=B:0,puts("|"))for(c=-1;c<w;c++)putchar(c<0?'|':a^c?b^c?32:B>0?92:47:b^c?A>0?92:47:88);}

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Kotlin, 322 311 302 bytes

Changed how I put laser direction in string for 11 bytes. Moved assignment out of when for 9 bytes.

{w:Int,h:Int,r:Int,l:Int->{var a=""
var f=r
var d=1>0
var s=l
var t=!d
for(o in 1..h){a+="|"
for(c in 0..w-1)a+=when{c==f&&c==s->"X"
c==f&&d||c==s&&t->"\\"
c==f||c==s->"/"
else->" "}
a+="|\n"
if(d){if(++f==w){--f
d=!d}}else if(--f<0){f=0
d=!d}
if(t){if(++s==w){--s
t=!t}}else if(--s<0){s=0
t=!t}}
a}()}

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