You might try emailing these guys and see if they have an eccentric BB that will work.
http://www.precisiontandems.com/catframepart.htm

I had an old cheap tandem I played with as a project for the grandkids I used an old RD I had around for a chain tensioner it actually worked really good and since it was spring loaded it required no adjusting ever.
Here is a photo if it more on my webpage it you want to see.