The first rule or tandem captaining is ” the stokers comfort is paramount!” The second rule is remember the first rule
Maybe drop the seat. get your weight back, heels down, bend your knees and elbows, relax?
The captain needs to call out whats happening. We have ridden a lot now and less is needed but I will still call “bump” for going up a step “jump” for going down a step “gears” for a big handful of gears especailly changing down and always for a front shift
The captain needs to not attempt to move the bike around under him – it needs to be ridden smoothly and you both keep hips and shoulders over the centre line – this makes the stoker feel more stable
Have a try in the other seat each – even if only for a bit on easy ground – it gives a good understanding of what is going on
I wonder if its a bit much too soon and thus you are tensing up too much.