Rolling resistance on tandem is higher (weight).(Riders+tandem+wheels).
Air resistance is higher : bar higher(also wider?) and frontal surface a bit higher, also frame tubes and other on tandem have a highe diameter.
U're more powerful than your wife : this means eg your max power 100% your wife 50%.
These added up and divide by two gives 75%.
To get 100% you will have to deliver 25% more power on the tandem .
These are my experiences.