It's much simpler, see https://github.com/microsoft/vscode/issues/142534

That is, you can add multiple --extensionDevelopmentPath in the launch.json configuration, e.g.

    ....
    "args": [
        "--extensionDevelopmentPath=${workspaceFolder}",
        "--extensionDevelopmentPath=${workspaceFolder}/../anotherExtension"
    ],
    ...