I have not come across a long tail cargo bike with a rear end tall enough to transport an adult. They are designed to haul kids who don't ride themselves yet or for cargo. The tail ends are low on purpose to keep the center of gravity low. This is why most long trail cargo bikes only use 20" wheels on the rear.
Why a cargo bike and not an electric tandem bike? This way you both have comfortable seats and can support each other pedaling.