I think it's a lot to do with centre of gravity. Mrs. Wow is tall (5'7") and with elegant long legs, therefore her saddle is high.
I've never had problems holding a tandem up even with a Rann trailer and a childseat, so an adult and two children behind. In that configuration S had to mount by standing on a pedal so as to swing a leg forward over the rear top tube. That did require that I had both feet on the ground.