Worry about communicating as a tandem team first
Before worrying about getting faster on a lighter tandem, worry about coordinating together and safety and overall happiness of the tandem team.
During our maiden voyage, we almost fell just descending our driveway (our driveway isn’t even steep). We also had trouble getting started on the bike after leaning at a standstill with our right feet on the curb. Shifting at the correct time without straining our gears while climbing uphill also took a while to adjust.
It probably took as around 500miles of road tandeming together in order to feel comfortable.
Because we ride a lot of hills (our typical ride is 35miles with 2500+ft elevation), we are probably slower by 1mph on the tandem than on our individual bikes on most rides. But we are faster by 1mph on a tandem compared to solo bikes if our route is really flat (75miles with 2000ft elevation).