Node js - Combine two streams in parallel way

I need combine two streams in parallel way. I want to use these streams to write into two files some generated information. I have done the first stream (stream1). It looks like this:

'use strict';
var fs = require('fs');
var stream1 = fs.createWriteStream("random1.txt");
let ln = 1000000;
let j = 0;

function generateRandom()
{
    var reslt = true;
    do {
        ln--;
        j++;

        if (ln === 0) {
            stream1.write(...write randon number...);
            stream1.end();
        } else {
            reslt = stream1.write(...write randon number...);
        }
    } while (ln > 0 && reslt);
    if (ln > 0) {
        stream1.once('drain', generateRandom);
    }
}

stream1.on('error', (err) => {throw err;});

var stream1Promise = new Promise((resolve, reject)=>{
    try {
        generateRandom();
        });
    } catch (error) {
        reject(error);
    }
});

stream1Promise
    .then(function(callback) {
        if (typeof callback === 'function') {
            callback();
        }
    }, function(error) {
        console.log("Error occurred: ", error);
        stream1.end();
    });

But I don't understand how to add another one stream stream2 so that it writes different random information to another file. I've tried to use process.nextTick in callback of stream1.write to switch my streams, but I get all time "FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - process out of memory" error. Could anyone help me? Thanks.

via Michiko