Yes, cgallagh can tell when I let up, and he can tell when I push the pace. On our ride today I did both: Pushed the pace +3 mph for a while, and I could tell that he wasn't ready for that, and when I was distracted by some crop dusters and unconsciously let up the effort he felt that, too.
As the stoker I can feel when he lets up and when he hammers. When I feel nothing I know we're in sync -- we're 90* OOP and for 95% of our rides neither of us have problems with the other being slow/fast. That's what keeps us returning to the tandem again and again.