You should read NOTES in this page. Here I copy and pasted it directly from that page:
NOTE: If you want multiple sections open at once, don't use an accordion
An accordion doesn't allow more than one content panel to be open at the same time, and it takes a lot of effort to do that. If you are looking for a widget that allows more than one content panel to be open, don't use this. Usually it can be written with a few lines of jQuery instead, something like this:
jQuery(document).ready(function(){
$('.accordion .head').click(function() {
$(this).next().toggle();
return false;
}).next().hide();
});
Or animated:
jQuery(document).ready(function(){
$('.accordion .head').click(function() {
$(this).next().toggle('slow');
return false;
}).next().hide();
});