dropper seatpost captain and stoker stem attached to dropper compatible?
 

Anyone already tried the combination of a 27.2 diameter dropper seatpost for the captain and a stoker stem attached to the dropper seatpost wit a shim?
Stem needs 4,5 cm, dropper about 6, seat post extension 25.5 cm. 25.5-4.5-6=15. So travel up to 15cm is possible.
But wil the force of tightening the stem to the seatpost hinder the seatpost in travelling up and down?
I suppose the force is not higher then the force needed for thightening the seatpostclamp.
If this is the case it should work.

I don't know how much tandem experience you have but the coordination with your stoker might be problematic. You drop the post and and all of sudden her (his) handlebars drop out from underneath them. Also the lateral torque of the stoker handlebars are loading up the seat post which may hinder function. We ride road, gravel and full suspension mountain tandems and never felt the need for a dropper post, just too many moving parts to coordinate . We have ridden some pretty crazy stuff in Moab just fine and we are getting up there in age so if we can do it a young team can fly through the stuff. Sometimes we clear stuff the singles struggle with as the tandem gaps stuff, obvious a skilled single can leave us in the dust.

I wouldn't know anything about it. And likely never will, we are a (primo) pavement only team. Still, post #2 sounds dead on. No dropper posts for the o.p. I am afraid. They are an answer to a question that few tandem Captains are asking.

The stem of the stoker is just above the seatpost clamp and under the top of the lower part of the dropper. So here handlebars don't move when the seatpost moves.
In technical stuff the fact that the center of gravity is lower and further back makes a difference when turning, dropping, riding over obstacles. That's also why i like to
ride the tandem front suspension with more sag. It makes i also easier to touch the ground with one foot. I don't think the lateral torque will be the problem as long as she's sitting down.
The torque of the stem could be a problem, but i think the torque o a seat post clamp is higher than the torque of the stem. I'm big(198) she's 1.52. The dropper makes it also easier
to make restart in a climb or a steep downhill.
And onroad I will go faster down in a more secure way. We ride a fullsuspension. We us more than 50 codes for warning her what to do or what's coming. The fullsuspension makes
a lot of things easier, more comfortable and more fun for her. The bike is also very stiff.

What you are describing sounds like a biological impossibility. But your mind seems made up. Go for it. Upload pictures. Like I said. This is outside my expertise but ... 50 codes? I've never counted, but I don't think we use more than 5. Are you SURE about how much fun she is having?

I've seen a few folks using dropper posts as you've described PNW, OneUp, and Thomson dropper posts all work with a stoker stem clamped onto the lower. Other brands probably do too.
https://cimg4.ibsrv.net/gimg/bikefor...9d09cbef54.jpg
Captain dropper seatpost on titanium tandem