I have ocassionally detected a strange problem with PHP sessions.
When I am running two PHP scripts using SAME session ID, second script is stuck until first one is comp
Yes, this is called "session locking" and is normal in PHP.
One solution is not not use sessions, just set cookies for your required persistent information.
Another solution is to implement your own session handler:
http://php.net/manual/en/session.customhandler.php
A detailed walkthrough about custom MySQL session handlers is here:
http://phpmaster.com/writing-custom-session-handlers/