ex2.js

var stream = require('stream');
var DuplexCombination = require('duplex-combination');

function dosomething() {
    var input = new stream.PassThrough();
    var output = new stream.PassThrough();
    var combined = new DuplexCombination(output,input)

    output.write('two');
    input.pipe(output);
    return combined;
}
 
var source = new stream.PassThrough()
 
source.pipe(dosomething()).pipe(process.stdout)
 
source.write('one')