It can be done, but you need to know the session key/ID - easiest way is to put the session key/ID as part of the URL (e.g. http://www.a-website.com?id=1a2b3c1a...1a2b3c1a2b3c4d).

You can then use this key in another browser to re-register the session using the existing session key/ID, using session_id() passing the session key/ID as the parameter.

However, revealing session keys/IDs as part of the URL isn't a good idea - goes against all security principles.