You can add a callback to the receive event and prevent users from putting more than 2 items into the same area. But this might prevent them from sorting if there's no empty space. And if they only have one empty space, it'll probably be a little bit painful to reorder the adoptables
HTML Code:
$('.sortable_adoptables')
.css('background-color', '#FFFF66')
.animate({ backgroundColor: realBg }, 'slow' )
.sortable({
cursor: 'pointer', connectWith: ".sortable_adoptables", receive: function(event, ui) {
var $this = $(this);
if ($this.children('table').length > 1) {
$(ui.sender).sortable('cancel');
}
}
});