running two Firebird servers 2.1 and 2.5 build 20334 simultaneously on one box, as applications. No environment vars apart from FIREBIRD_MSG, no registry entry. Firebird 2.5 has been given port 3055 in firebird.conf.

Firebird 2.1is started through its guardian by running 'fbguard -a'. Trying to do the same with the guardian of the other instance gives

The firebird guardian failed to startup because another instance of the guardian is already running.

Using -s (or -n) <instance_name> when starting the guardians makes no difference.

Also, pointed out by Vlad, switch -a must be set last else it is ignored.

Running both instances works if Firebird 2.5 ist started by running fbserver -a

Volker