Hi Jose,
I recommend looking into using the Multi Event Registration add-on. In your case, you may want to set up the event registration to flow through the cart registration as outlined in this guide: http://eventespresso.com/support/documentation/multiple-events-registration-add-cart-link-default-registration-form/
The Multi Event Registration add-on can be purchased separately to use with the Basic version. It is bundled with the Advanced license: http://eventespresso.com/download/