I can't now remember how we got the stoker bars high enough for me on LEJOG, but we didn't find a useful product and it was bodgetastic. Extendable stems bring the bars towards you as well as up, which wasn't helpful. A long-reach adjustable stem clamped onto the stoker stem, maybe?
Definitely I used flat bars with bar ends. With any stoker stem you can of course easily exert enough torque to twist the pilot's saddle round -- so you don't. It didn't matter that the arrangement we ended up with was a bit flimsy.