A continuum theory has been developed to give account of the three characteristic zones appearing in the "semisoft" behaviour of nematic single crystal elastomers under the effect of biaxial mechanic and electrical force fields considering higher order coupling terms. The analysis is based on a model which takes into account two coexistent coupled components showing a preferred direction. Two cases are analysed according to the type of input: stress and displacement. Bifurcation phenomena are predicted in the first case. Sress-strain curves show the characteristic features experimentally observed. The case where shear terms in the strain are considered is approximately analysed.