If the cookie change, that should be two complete different sessions, or do i miss a thing?
So we only need to know that two log entries have the same cookie, but do not know what value the cookie had have.

Another point is, all cookies of a request are stored in a string like they are transmitted in the request.
So if the session cookie is the same as in another but there is a second cookie, the hash changes completly.
It is debatable if we should parse the string, seperate the cookies and hash each on it own.