I've added a second (mobile)menu to a Wordpress theme, it works fine, but I know I could of done better with the javascript. I've very new to javascript.

Basically I need to add a second ID to getElementById

to get around this, I've just copied the entire js and changed the ID and pasted it at the end of the file. I'm sure there is a better way to do this.

This is the section I need to change the 'site-navigation' part, I need it to also include 'lower-navigation'

Code:
var button = document.getElementById( 'site-navigation' ).getElementsByTagName( 'h3' )[0],
	    menu   = document.getElementById( 'site-navigation' ).getElementsByTagName( 'ul' )[0];
Here is the full .js file http://pastebin.com/PmAsjUg7