<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
</head>
<body>
<div class="sepSt">
<div class="waper-catalog-row-col-3">
<span>text</span>
</div>
<div class="waper-catalog-row-col-4">
<span>другой text</span>
</div>
</div>
<div class="sepSt">
<div class="waper-catalog-row-col-3">
<span>text</span>
</div>
<div class="waper-catalog-row-col-4">
<span>другой text1</span>
</div>
</div>
<div class="sepSt">
<div class="waper-catalog-row-col-3">
<span>text</span>
</div>
<div class="waper-catalog-row-col-4">
<span>другой text2</span>
</div>
</div>
<script>
$('.waper-catalog-row-col-3').each(function(){
$(this).append($(this).siblings('.waper-catalog-row-col-4').children('span'));
});
</script>
</body>
</html>
так?