I have a Bernina 710, which has the built in dual feed and I can tell you, the real walking foot is necessary for more than two layers. Bernina recommends use of the walking foot for, "more than two layers".
The Bernina dual feed LOOKS just like the Pfaff dual feed, but it is not like it. I don't know why....an engineering mistake...????, or on purpose so we would still have to pay $150-$200 for the Bernina walking foot.