Is it acceptable asking to join a researcher in co-organizing their conference session?
I would not directly broach the subject. If you want to judge interest, the kinds of questions I'd ask are:


  What do you have to do to organize a session?


If your friend mentions needing a co-chair, you could ask your friend if he has someone in mind. 

But do not ask directly to be a co-chair for someone else's session. What you might instead want to ask is: "would you like to be co-chairs for a future session sometime?"