I second DieselDan, We bought one new, and it pivots on the seatpost. I don't know how you could ride one without the proper plastic spacer.
They have to be pretty tight or they wobble. Ours sorta "creeked" a lot. I always wondered about greasing the seatpost, but never did. Ours disappeared out of the garage shortly before someone offered to sell us a used Burley. The Burley is certainly nicer to pull, with its rack mount.
I emailed Trek support when it was fairly new to ask how to properly tighten it, and never received a reply. but with practice, trial and error, came the realization that it had to be tight.
I always wanted to design a hitch for these type of bikes that leaned the tandem as it pivoted. My daughter did fine until she became a good rider on her own, and then always complained that it felt like it was leaning the wrong way when turning. Or is it just the lack of control that scared her?