I must emphasize that my investigations took place years ago, on April, 2013 so during this long time Ambari must have been updated a lot as it is shown by its GitHub repository. The latest version of Ambari is 2.1.2, released two days ago, but mine could not be more recent than 1.2.2.
If everything went fine you should be able to define which cluster to show by adding an extra HTTP GET variable to the URL of the web UI, like this:
That's all and I hope it works. If you have any further hands on experience please feel free to contact me and I will update this post.
There are absolute no guarantees that this will work so do it at your own risk!