Our Burley tandem is a L/M (20.5/17). A M/S might have fit me and my wife better but we've made it work. For the last year, though, since about the same time kiddo 1 got his 16er, it's had a kid back. This has been a lot of fun but it can be a little hokey - the boy is up high and it's a lot of moving parts. Installing the thing created some problems with the BCD and I've gone 1x just to get rid of the front stuff going on around his feet.

I got a set of crank shorteners by chance a few months ago and just decided to measure it out to see if he's ready for that. His 16er is 20 inches from down pedal to seat. But the tandem is 17" in the rear... with 4in of crank and a couple inches for the seat he would need to grow 2-3 inches of inseam. So we can't do that yet for a few years . Kids 2A and 2B will be ready for the kid back next year, too, so more reason to leave it on.

What I might do in the medium term - like a year from now - is get a newer MTB tandem in size M/S that has a lot more standover and therefore a shorter rear seat tube... for instance a Fandango is 15.5 in the rear. Then I can put kiddo on the main cranks with the crank shorteners (mom too without them) and still be able to mount the kid back for a while longer, or maybe just keep the Burley a while longer, for kids 2A&B.