A 'bakfiets' is a Dutch adaption for converting a bike into a cargo bike for hauling stuff or people.
Translated from Dutch: bak = box and fiets = bicycle.
Sounds like StephenH is interested in converting a tandem into a transporter bike.
Bakfiets is either a 2 or 3 wheeled bike with a box up front; looks like he wants to put the cargo box in rear of that tandem.