Was just going to make the same observation as zonta. I don't have a lot of time on a tandem, but do have 30+ years on the road (and an engineering degree ). I noticed this same effect riding with my then 11 year old son. I think the OP was on the right track, but the root cause I believe has more to do with your stoker having a flat spot in his/her stroke that coincides with yours. My son actually grew out of this, funny I can feel a lot of push coming from the back now if I ask for it.

Like the idea of trying OOP.