You need two of the tagalong units and a friend of your kid. Power meters installed on both units. Screens for both power units on both bikes. And psychology. Get them to compete. He who does the most work wins. (I'd keep the competition down to just the uphills - thinking young attention spans. House at the bottom to sign at the top.)

This could be an excellent reason for future parents to consider improving the odds for twins. Now you have in-house competition (actually, like it or not!). But on the bike, that could be very useful. My best friends and cycling buddies were twins but we did not meet until we were 10. No way were we riding with mom!