var file1 = fs.createReadStream('...');
var file2 = fs.createReadStream('...');
var ven = require('venstream');
 
var union = fs.createWriteStream('union.out');
 
var handles = ven(union);
file1.pipe(handles[0]).pipe(fs.createWriteStream('file1only.out');
file2.pipe(handles[1]).pipe(fs.createWriteStream('file2only.out');
 
union.on('finish', function () {
  console.log('Stream Processing Finished!');
});