加入会话ID

I have a login area for my customers. They send me data using there userid as a ref and they can login and view what they have sent.

Now some of my customers have more than one website and more than 1 userID.

Example :

Martin's userid = MW001 for website www.martin.com Martin's userid = MW002 for website www.thomas.com

Now once they login , there user id is stored in a session which i use to display data from a mysql DB.

Is there a way to store both userid's in a session and display different information based on what they choose.

Example :

Martin logins in using userid MW001 for website www.martin.com , he can now view all the data related to userid MW001

I want him now to have a option to select a link called " View other website " and it will load that data for userid MW002 and not MW001 anymore.

My DB Looks Like This :

userid , Name, Surname, Email, Tel, URL

OK , what i need to know.

I need to know how to move between sessions.

If my customer logins in user userid MW001 how will i load MW002 ?

What i did was just create a session, then join mysql tables and link the users that way. It was actually really simple.