To accomplish what you are wanting to do, it is possible to use the FEDERATED storage engine on both servers, in conjunction with triggers, to allow each server to update the other server's database.

This is not exactly a simple out-of-the-box solution, because it requires additional precautions and requires you to decide whether consistency or isolation tolerance is more important and allow the queries to fail when the other server isn't available (more consistency) or use a CONTINUE HANDLER to suppress errors (isolation tolerance).

But here is an extremely simplified example.

Each server would have the identical configuration.

The local user table:

CREATE TABLE user (
  username varchar(64) NOT NULL,
  password varbinary(48) NOT NULL, /* encrypted of course */
  PRIMARY KEY(username)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

A local table that is federated to the user table on the other server.

CREATE TABLE remote_user (
  username varchar(64) NOT NULL,
  password varbinary(48) NOT NULL, /* encrypted of course */
  PRIMARY KEY(username)
) ENGINE=FEDERATED DEFAULT CHARSET=utf8 CONNECTION='mysql://username:pass@the_other_host:port/schema/user';

Selecting from remote_user on one server will retrieve the records from the other server, and insert/update/delete on that table will change data on the other server.

So, we create triggers do accomplish the purpose of updating the distance server. They are written as BEFORE triggers, with the idea being that we don't want to do something to ourselves that we can't do to the other server -- for example, if a username already exists on the other server, but not here, we want the insert on the other server to throw an error that prevents us from creating the user here... as opposed to creating a user here with what would be a conflicting username. This is, of course, one of the tradeoff decisions you'll need to make.

DELIMITER $$

CREATE TRIGGER user_bi BEFORE INSERT ON user FOR EACH ROW
BEGIN
  INSERT INTO remote_user (username,password) VALUES (NEW.username,NEW.password);
END $$

CREATE TRIGGER user_bu BEFORE UPDATE ON user FOR EACH ROW
BEGIN
  UPDATE remote_user 
     SET username = NEW.username,
         password = NEW.password
   WHERE username = OLD.username;
END $$

CREATE TRIGGER user_bd BEFORE DELETE ON user FOR EACH ROW
BEGIN
  DELETE FROM remote_user
   WHERE username = OLD.username;
END $$

DELIMITER ;

This is not a perfect solution and is not a high-availability solution, because it relies on solid connectivity between the two systems and even if you are using InnoDB and transactions, the actions you take against the target table are not part of your local transaction and cannot be rolled back.

I use the FEDERATED engine quite a bit; it comes in handy for a number of creative purposes in my environment, including one situation where I used a federated query launched by a trigger to impose foreign key constraints against a foreign data source; however, I restrict its use to back-end processes where unexpected issues such as timeouts, coding errors, or server-to-server network/outage/isolation events cannot result in the end user on one of our web sites experiencing any kind of problem. Your ability to tolerate such a situation would be a major determining factor into whether this is an appropriate solution.

An alternative would be to configure your two servers in master/master replication. For this, you would need to use different database names on each server, so that for most events that replicate, the two servers could not possibly conflict with each other. In the worst-case scenario, if you lose connectivity or encounter a replication error, the two sites would still be running independently and you could resynchronize and recover. Configuration would look something like this:

database_a database for site A
database_b database for site B
database_c database for only the shared table(s)

Then, in database_a and database_b:

CREATE ALGORITHM=MERGE SQL SECURITY INVOKER VIEW user AS SELECT * FROM c.user;

MySQL will treat database_a.user and database_b.user as aliases for the "real" user table, database_c.user, so you would not have to change your application other than to use its designated database (i.e, you wouldn't have to configure it to understand that the user table was actually in a different schema, because the view will function pretty much transparently with this configuration). If the schemas have foreign keys against the user table, you would declare those against the true base table database_c.user.

Configure the two servers to replicate everything, but set auto_increment_increment and auto_increment_offset appropriately so you do not have conflicting auto-increment values on the shared table(s), if your tables use auto-increment. (Note, the documentation says that these variables are for NDB tables only, but that's not accurate).

An extra advantage of this setup is that your two servers would then have a complete duplicate of the other site's data that you could potentially use to your advantage for recovery from hardware failure in one of the servers.

This might be a good case for using an update trigger.

Look at some of the examples here: http://dev.mysql.com/doc/refman/5.5/en/trigger-syntax.html

What you'll basically do is create a trigger on the first table that iterates a matching row in the second table when a row is added to the first table (on insert).