Thanks for the hint! I think I've found a way by assigning each #dropMenu an individual id# in tpl_drop_menu.php like this:

<div id="dropMenuWrapper">
<div id="dropMenuWrapperb">
<div id="dropMenuWrapperc">
<div id="dropMenuWrapperd">
<div id="dropMenu">
<div id="dropMenu1">
<ul class="level1">
<li class="submenu"><a href="<?php echo zen_href_link(FILENAME_DEFAULT); ?>"><?php echo HEADER_TITLE_CATALOG; ?></a>
<ul class="level2">
</ul>
</li>
<div id="dropMenu2">
<li class="submenu"><a href="<?php echo zen_href_link(FILENAME_SITE_MAP); ?>"><?php echo HEADER_TITLE_CATEGORIES; ?></a>
<?php
This seems to work.. (I need to learn some php coding though...)