node.js : function.prototype.method multiple exports inside one file

Noob in JS here.

Let’s say I have a js file where the contents are like

function a(){}
a.prototype.method1 = function() { ... };
a.prototype.method2 = function() { ... };

function b(){}
b.prototype.method3 = function() { ... };
b.prototype.method4 = function() { ... };

module.exports = new a();
module.exports = new b();

In this file, I’ve function definition.
In the next file, I’m trying to access the methods below

var x = require('./file.js');

x.a.method1();   //Not working
x.method1();     //Not working

But both types of accessing result in error for me. Can you please help me with steps on how to access the methods correctly?

And I can access those functions if I try to have only 1 export in one file. But I want it to be like this. 2 exports in a same file.