We are running a setup with one author instance and 2 public instances. For the use of commenting we
have clustered the forum workspace. This means sthe repository is split using the repositories.xml, which
specifies which workspace goes where. However when we run the backup using the backup module it only
backs up the default magnolia repository and not the magnoliacluster repository which we also have.

We do get a backup of the forum workspace, but it is an empty one which probably gets created on
bootstrapping.

Workaround
Use backup extended for multiple repository support.