Fallback mounts are handled directly by icecast, so there is no additional configuration required in Airtime. It is done by setting up fallback-mount in your icecast.xml file. If you read the man page for icecast it gives you the full range of options you can specify for this. Essentially you have a 3 tiered system of mounts in your icecast configuration. The main mount point that your listeners connect to, this should always have no content at all streaming directly to it. You then set up another mount point that Airtime will stream on to fall back to this. On top of that you set up a live mountpoint that will always override the Airtime stream.
I will try and post an example when I am near my laptop, though this won't be for a few days.
If you are talking about running several instances of Airtime without virtual machines, then currently this is not possible. This is something we expect to fix in the near future.