I did install a standalone HSQLDB and two times Openfire which share the same database but it seems that the "DB Connection URL:" "jdbc:hsqldb:hsql://localhost/cluster" or the HSQLDB driver is recognized as an internal database. So Openfire displays "Clustering is not available when using an embedded database. You need to switch to an external database to use clustering." when I try to enable clustering.
Everything went fine so far, one of the server logs a lot of errors and warnings with "Sample time of 12345 for statistic XYZ is invalid." but this seems to be normal as they share one database.
While I like to use a lightweight database for tests I wonder if this is possible.
I got a fix from Gato, now it's possible for me to use the HSQLDB (still not in embedded mode but as a standalone database).