Perhaps you can put the 2nd WO after the pause point of the button, then set the button action to continue. You can't control the visibility of an object opened in a separate tab.
Use the Timeline: stagger the webobjects on that timeline so that the second object is starting later. Move the pausing point of the button so that it is slightly before the start of the timeline of the second web object. Or use micronavigation.
First of all, as an FYI - opening in a new window will trigger a pop-up window. Which may or may not get blocked by your user’s browsers.
Embeds are more tricky - the easiest would be instead of using the embed code, to use a direct link to the video? or other thing you are embedding.
Otherwise you need to create an html page with the embed code. include the html file in your manifest and then in window.open
add the html file name and any folder structure if it is not on the same folder leve as your index.html file