@mysterious-toddler-20573 - Could you please try it and give feedback before I send a pull request to add this extension? I will also prepare the content for the documentation page. Below is simple test script.

php
<?php
$div1 = rand(0,99999999);
$div2 = rand(0,99999999);
$div3 = rand(0,99999999);
$random = rand(0,99999999);
?>
<html>
<head>
    <title>Page <?php echo $random; ?></title>
    <style type="text/css">
        div { clear: both; display: block; }
    </style>
    <script src="htmx.js"></script>
    <script src="htmx.multi-swap.js"></script>
</head>
<body hx-boost="true">
    <h1>Heading (<?php echo $random; ?>)</h1>
    <a href="index.php" hx-ext="multi-swap" hx-swap="multi:#id1,#id3:beforeend">Refresh</a>
    <div id="id1"><?php echo $div1; ?></div>
    <div id="id2"><?php echo $div2; ?></div>
    <div id="id3"><?php echo $div3; ?></div>
</body>
</html>