Check if the user has scrolled past the header ad, then display the footer ad.
if($(your header ad).position().top < 0) { $(your footer ad).show() }
Am I correct at what you are looking for?
See jQuery.scroll(). You can bind this to the window element to get your desired event hook.
On scroll, then simply check your scroll position:
$(window).scroll(function() {
var scrollTop = $(window).scrollTop();
if ( scrollTop > $(headerElem).offset().top ) {
// display add
}
});
Source: Stackoverflow.com