Is is possible to access the same DBMS from multiple Hibernate instances possibly running on another system?
What implications does this have? I could imagine Hibernate events not to be working anymore.
What is a good approach to high availability?

yes

the implications are the same if you were using jdbc. reading should be just fine but when it comes to updating...

either you'll have very complex code or you'll get numerous errors if you try to update rows if they've been changed by another application