  In this paper we consider $(x,u)$-flat nonlinear control systems with two
inputs, and show that every such system can be rendered static feedback
linearizable by prolongations of a suitably chosen control. This result is not
only of theoretical interest, but has also important implications on the design
of flatness based tracking controls. We show that a tracking control based on
quasi-static state feedback can always be designed in such a way that only
measurements of a (classical) state of the system, and not measurements of a
generalized Brunovsky state, as reported in the literature, are required.
