The dielectric constants of $(RDA)_{1-x}(ADA)_x$ mixed crystals are measured along the a- and c- axes for two values of x, x=0.51 and 0.35. The measured dielectric constants show considerable dependence on concentration and axial directions. $(RDA)_{0.49}(ADA)_{0.51}$ undergoes AFE phase transition with Tm=96K and $T_N$=83K, while $(RDA)_{0.65}(ADA)_{0.35}$ undergoes a proton-glass phase transition. We confirmed that the phase diagram of $(RDA)_{1-x}(ADA)_x$ was not symmetric with respect to ammonium concentration x.