Override session id creation and cookie creation?

In my module, msession, I have routines that guarentee that a session id is unique within a cluster of web servers. The problem I have is that session creates and sets the session cookie without some mechanism to allow me to replace the session id string, prior to the session code setting the cookie. In some test code I have, I set the PHPSESSID cookie with my value, and replace the internal value used, but oddly enough both values exist in the browser, and depending on any set of random events, it is not predictable which PHPSESSID value gets used. Any hints? Tips?