Burley Softride tandems had braze-ons to attach a standard adjustable rear rack as did most Co-Motions.
Burley sold metal extensions for the rack to reach the x-tra length to the brazeons.
If you cannot find rack extensions, should be easy enough to frabricate something.
A beamed tandem weighs in a few pounds more, but what is stoker's comfort worth? Then S&S also adds additonal weight, but what is that convenience worth to you when you travel? Would guess your Co-Mo weighs +/- 44 lbs.
Have actually seen a couple custom tandems with beams for pilot and stoker.
When you think of it, captain really does not need a beam as he/she is sitting in the middle of the tandem frame.
But when you go cu$tom, you get what you want/need, even a double-beemer!
Enjoy the ride TWOgether!
Rudyand Kay/zonatandemn