This is not yet documented, but as I was updating my gulp task script for changing environment settings, I wanted to test things out.
The process of creating multi-component bundles seem fairly easy. Since the v1.2.0 release of SharePoint Framework, the config.json structure got changed.
The old structure
In the old structure, the entries property contained an entry per asset you were building. For example, a web part or extensions.
The new structure
In the new structure, it looks like this:
Info: When you want to migrate an application from an older version to the current v1.2.0 one, you will have to update the config.json file. Luckily there is a handy command for this: gulp --upgrade. Once you have ran this, the config.json structure gets updated to version 2.0. Mikael Svenson wrote an article about the upgrade process which you can read here: http://www.techmikael.com/2017/08/how-to-update-sharepoint-framework.html
What you can see in the updated config file is that the entries section has been changed to a bundles section. By default, for every component, a new bundle will be created, like how it was with the previous configuration file.
So, if we would add a couple more components, we get the following output:
Making your own multi-component bundles
In order to make multi-component bundles, you will have to add your component entries to a specific bundle of your choice. Like you can see here:
What I did in this bundle is combining the hello-world-web-part and second-web-part in one bundle called the multi-webpart-bundle. When you now bundle your project, this will create two bundles:
At the end of the multi-webpart-bundle.js file, you can see that it exports two components:
These IDs correspond to the IDs in the manifest of the web parts. When you test the web parts in the workbench, you will only see that one bundle file being loaded for both web parts:
Info: this was just an article to show you what you can expect and how it works. The documentation around this topic will probably be released very soon.