You must create the div in the same html, with different ID name, then by jquery, you can indicate through the button the div to show or to hide.
HTML
<div class="btnmenu" id="menua">
<div style="cursor:pointer;border:none;"><h2><img class="img" id="imga" src="imagenes/mas.png"/><h2> TITULO</h2></div>
</div>
<div id="obsa" class="obs" style=" display:none;">
xxxxxxxxxx
</div>
<div class="btnmenu" id="menub">
<div style="cursor:pointer;border:none;"><h2><img class="img" id="imgb" src="imagenes/mas.png"/><h2> TITULO</h2></div>
</div>
<div id="obsb" class="obs" style="display:none;"> xxxxxxxxxxxxxxxx
THE JS would be
$("div.btnmenu").click(function(){
id=this.id.substr(4, 1);
valor=$("img#img"+id).attr("src");
$("div.obs").slideUp("normal");
$("img.img").attr("src","imagenes/mas.png" );
$("div#obs"+id+":hidden").slideDown("normal");
if (valor=="imagenes/mas.png")
{
$("img#img"+id).attr("src","imagenes/menos.png" );
}
else
{
$("img#img"+id).attr("src","imagenes/mas.png" );
}
});