main.js

const electron = require('electron');
const windowManager = require('electron-window-manager');
const path = require('path');
const url = require('url');

const app = electron.app;
const BrowserWindow = electron.BrowserWindow;

const windows = [];

function createWindow() {
    const width = 300;
    const height = 600;
    let w = new BrowserWindow({
        width,
        height
    });

    w.loadURL(url.format({
        pathname: path.join(__dirname, 'index.html'),
        protocol: 'file:',
        slashes: true
    }));

    const x = windows.length * width;
    const y = 0;
    w.setPosition(x, y);
    windows.push(w);
}

app.on('ready', function() {
    createWindow();
    createWindow();
});

app.on('window-all-closed', function() {
    if (process.platform !== 'darwin') {
        app.quit()
    }
});