Why would you want to connect two different users of a database together?
I've been using databases professionally for many years and I don't recall
_ever_ needing to have two different database users connected together. It's
common to have many users accessing the same database at the same time but
they are each communicating with the _database_ NOT with each other.

If you really want two different users to communicate with one another while
they are using a database, wouldn't it make more sense for them to chat with
one another (by voice or chat program) rather than to use JDBC? I could
imagine two users needing to talk to one another while collaborating on a
database update. For example, one user might be the teller at a bank and
he/she might need to talk to a supervisor about some aspect of the update
while it is taking place: the teller might want the supervisor to be able to
see the data for that customer at the same time and the supervisor might be
at a different location (working at home maybe) so that both need to be
signed on separately to see the data. The teller might ask some questions
and get some advice from the supervisor but I would imagine that they would
communicate by voice or chat, not via JDBC. Each person would access the
data independently by JDBC but they would communicate with each other via
different means.

--
Rhino