data-show-ajax=".block1"

$('[data-show-ajax]').on('click touch', function(event) {
  event.preventDefault();
  $($(this).attr('data-show-ajax')).show();
  $(this).hide();
});
if ($("div.block1 .block-cnt").length < 15) {
	$("button[data-show-ajax='.block1']").hide();
}

<div class="block1"> {custom category="2" template="shortstory" order="date" sort="asc" limit="15"}</div>
<br />       
<button style="width: 100%;" data-show-ajax=".block">Показать все</button>
<div class="block" style="display:none">
{custom category="2" template="shortstory" order="title" sort="desc" from="15"}
</div>