On our one TT effort, we had a pair of those stoker clipon aero bars
pictured in the dual ladies tandem, but on our bike the setup was such
that the bars were really close to the back of the pilot's thighs and
weren't feasible to install. Pilot has progressively moved his set up toward
the rear as time goes on and aerobar pushes him a bit further back.
We have noticed that pilot on the aero
bars is good for about 3mph increased speed at 30+ mph for the same
pedaling effort compared with the pilot on the drops or hunched a bit
on the shifters. Problem for the pilot is how long they can stay on
the aero bars-some people have breathing problems at high effort in
the aero position.

re HRM, we both have one but mine reads out the pilot HR when on the
tandem, so I have an excellent idea of where the pilot is in his effort
curve.