Stroke and other neurological disorders often lead to reduced motor function and to pathological foot motion during gait. We consider Functional Electrical Stimulation (FES) of the shank muscles that control dorsiflexion (related to pitch) and eversion (related to roll) of the foot. We describe the nonlinear domain of stimulation intensities that are tolerated by subjects in combined two-channel FES via surface electrodes. Two piecewise linear parameterizations of this domain are suggested and compared in terms of the cross-couplings between the newly defined stimulation intensity coordinates and the foot motion caused during swing phase in drop foot patients walking on a treadmill. Both parameterizations are found to yield almost monotonous input-output behavior and therefore facilitate decentralized control of the foot pitch and roll angle.