Fixed version of Unity WebGL dual build editor script example (https://docs.unity3d.com/2022.2/Documentation/Manual/webgl-texture-compression.html) which lets you make WebGL builds with both DXT and ASTC compressed textures... Don't forget to update your WebGL template HTML too.

BuildWebGLDual.cs

using System.Collections;
using System.Collections.Generic;
using UnityEditor;
using System.IO;

public class BuildWebGLDual
{
    [MenuItem("Build/WebGL Dual Build")]
    public static void BuildGame()
    {
        // based on https://docs.unity3d.com/2022.2/Documentation/Manual/webgl-texture-compression.html

        // This builds the player twice: a build with desktop-specific texture settings (WebGL_Build) 
        // as well as mobile-specific texture settings (WebGL_Mobile), and combines the necessary files into one directory (WebGL_Build)

        // Don't forget to edit your WebGL player template HTML to choose the right data bundle.

        string dualBuildPath = EditorUtility.SaveFolderPanel("Choose location of WebGL builds", "", "");
       
        if (!Directory.Exists(dualBuildPath)) {
            Directory.CreateDirectory(dualBuildPath);
        }

        string desktopBuildName = "WebGL_Build";
        string mobileBuildName = "WebGL_Mobile";

        string desktopPath = Path.Combine(dualBuildPath, desktopBuildName);
        string mobilePath = Path.Combine(dualBuildPath, mobileBuildName);

        string dataFileExt = ".data";
        switch(PlayerSettings.WebGL.compressionFormat) {
            case WebGLCompressionFormat.Gzip:
                dataFileExt = ".data.gz";
                break;
            case WebGLCompressionFormat.Brotli:
                dataFileExt = ".data.br";
                break;
            default:
                break;
        }

        var buildOptions = new BuildPlayerOptions();
        buildOptions.scenes = GetScenePaths();
        buildOptions.target = BuildTarget.WebGL;
        buildOptions.options = BuildOptions.None;

        // build mobile
        buildOptions.locationPathName = mobilePath;
        EditorUserBuildSettings.webGLBuildSubtarget = WebGLTextureSubtarget.ASTC;
        buildOptions.subtarget = (int)WebGLTextureSubtarget.ASTC;
        BuildPipeline.BuildPlayer(buildOptions);

        // build desktop
        buildOptions.locationPathName = desktopPath;
        buildOptions.options = BuildOptions.ShowBuiltPlayer;
        EditorUserBuildSettings.webGLBuildSubtarget = WebGLTextureSubtarget.DXT;
        buildOptions.subtarget = (int)WebGLTextureSubtarget.DXT;
        BuildPipeline.BuildPlayer(buildOptions);

        // Copy the mobile.data file to the desktop build directory to consolidate them both
        FileUtil.CopyFileOrDirectory(Path.Combine(mobilePath, "Build", mobileBuildName + dataFileExt), Path.Combine(desktopPath, "Build", mobileBuildName + dataFileExt));
    }

    static string[] GetScenePaths()
    {
        string[] scenes = new string[EditorBuildSettings.scenes.Length];
        for (int i = 0; i < scenes.Length; i++)
        {
            scenes[i] = EditorBuildSettings.scenes[i].path;
        }
        return scenes;
    }
}