Is this on the same domain? If these are not on the same domain you will not be able to share sessions, as the session cookies can only be set to a specific domain/subdomain, or to any subdomain of a single domain. But they cannot be shared across domains as that is not allowed by browsers.
If this is a subdomain, you can set a system setting, session_cookie_domain, with a value of:
The period at the beginning will allow you to share the session cookie across any subdomains you have set-up as Contexts.