""Do you find that your HR is lower on the tandem with the same workload during a TT VS single TT,""


NO...
Generally, when fitness level is good and training has been going well, if I have a session when my HR won't come up as you just described...(regardless of what I'm riding or even running for the matter) it is almost always because I've not recovered from a prior hard effort. In short, I suspect the long effort from the night before cooked you and you did not have time to recover.

...if you are on form and rested, even if the stoker is not, your HR should respond favorably (rise and fall quikckly) to your hard/easy efforts. Of course the team's overall performance/speed will be off because one team member is off her game and it will certainly feel like extra work for you, but independent of that, our HR will respond to our condition and effort.

Bill J.