SJX426 

I would be concerned about the potential of the rear bouncing. The strap that ties the frame to the front bracket helps but you can't tighten the strap to a point where it isn't elastic enough to allow movement. The tire pads reduce the force on the surface but does not hold the rear of the bike down. You could add weight to the rear of the bike but the bounce could still happen. My preference would be to find a way to hold frame down to the surface, the further back the better.

The only restraint keeping the bike from flopping on its side is the force of the QR on the front fork. There are two joints where the bike can rotate, the head tube and the front axle. Bouncing the rear wheel could cause the rear to move side to side with the rotation happening at the head tube and the front axle. A Tandem has a very long moment arm (lever) from the rear axle to the front. If the QR isn't tight enough, you will get movement and it may "rock" itself out of the bracket.