Advanced planar MOSFET and FinFET transistors on SOI have been characterized under high magnetic field. The geometrical magnetoresistance stands as the most accurate and indisputable technique for mobility measurements. Our results show that this method is also effective in both planar (FD-SOI) and vertical (FinFET) transistors with ultrathin body. For the first time, we apply the magnetoresistance for evaluating not only the properties of separate channels, but also their interaction mechanisms. Unconventional mobility curves with multi-branch aspect are recorded when two or more channels coexist. They are explained by the variations in effective field and centroid of the inversion charge. A marked difference is observed between front and back channels as well as between planar and FinFET devices.