Yes. Have a closer look at the cookie specification. There is a link to it on the setcookie [php.net] function page. Actually, it is the domain argument on the PHP manual page there that describes how you can overcome your issue.
I'd highly recommend approaching this problem from a different direction... Standardize your site on one version or the other and redirect people to the correct version. Not just for session control, but also to keep from splitting your inbound link juice across multiple domains and to minimize duplicate content.
Add code similar to the following to the top of each page (or hopefully you have a global include file you can add this to):