Just reminds me of a Youth Hostel Tandem trip we did around North Wales, when I was in my teens.
I had to re-do the brakes before the trip and could not get a long enough brake cable to reach from the front to the back wheel. So I used an ordinary cycle cable and put the rear brake lever on the rear handlebars... It all worked Ok as long as P1 instructed P2 when to apply the rear brake.
However we got to a long steep mountain pass, and the speed steadily increased... We zoomed past several motorists, then saw a sign saying 'Beware Steep Hill 1:6 ahead'. I said 'Pip, put on the rear brake'.. he said it had been on all the way... OOOH... So, as we had lost the front mudguard previously, I applied my foot to the front tyre and we came to a smokey halt just before the very steep bit., and the two cars overtook us once more.
Perhaps if you were to control the Right Rudder whilst your mate had the Left one, then that would count... but maybe not.