Can several instances of logFaces server share a database?

Yes, but only if you use external database. Primary keys are generated by database and all operations are transactional, so several servers could work safely with the same storage.

You should be more cautious with this type of configuration, particularly make sure that database capacity is configured in both servers to an identical value. Also, publish schema manually to your database instead of relying on either instance of the server to do this automatically because it might result in race conditions during first time start-up.