Session ?

Something new for me - working with scripts on two of my sub-domains. I want to call script 2 in my B domain from script 1 in my A domain. It appears that the session vars established in script 1 do not show up in script 2. Is that because the domain name is different? Is this where one must grab the session id and use it in another script in order to retain the original session and its vars? If this is so, can someone elaborate on the capture of the session id process?