This work reports experimental confirmation of the working principles of a double magnetic tunnel junction (DMTJ) to achieve highly efficient spin-transfer-torque (STT) switching. This concept uses a magnetically switchable assistance layer (ASL) acting as top perpendicular spin polarizer. The STT-induced switching dynamics were described by macrospin simulations while the magnetic and electrical properties of the device were investigated in switching experiments. The reversal dynamics of the storage layer-ASL coupled system were validated by time-resolved measurements of the device resistance during write operation, confirming that the storage layer is subjected to additive STT contributions both from the reference layer and the ASL throughout its parallel-to-antiparallel and antiparallel-to-parallel transitions. The STT efficiency of the ASL-DMTJ was compared to that of single MTJ stack comprising the same storage layer and no assistance layer. The figure of merit ∆ /Ic (stability/critical current) was found to nearly double in devices of 80 nm and 100 nm diameter, with a smaller 30% increase obtained for 50 nm diameter cells.