Check out this Moulton APB Tandem on Youtube.
Moulton have never built a factory tandem, but several Mouton based tandems have been created as custom projects, by third-party framebuilders.
Several APB based tandems have been built by Doug Pinkerton. Here is one example of a Doug Pinkerton Moulton APB Tandem
See also an ATB based tandem in Australia
All these tandems I know are based on a Moulton ATB, not an APB.