Thank you.
You are quite right - it is not necessary.
I finally opted for the single session and deregistering session variables individually rather than destroying the whole session (which seems a much more sensible way of doing things that I just did not think of before - duh).
So I don't know if two simultansous sessions works yet as I don't have time to try it but I feel that it won't work as will the browser use two session id strings in the URL if need be?
If anyone knows it would satisfy my curiousity if you could post an answer here!
Thanks
Mark