Mixed crystals of $Rb_{1-x}(NH_4)_xH_2P_{1-y}As_yO_4$(RADPA-xy) and $Cs_{1-x}(NH_4)_xH_2AsO_4$(CADA-X) are studied for various mixing concentraions (x,y)by Raman scattering and dielectric measurements in the temperature range from temperature to T=15K. Low temperature Raman spectra of RADPA(x=0.55, y=0.65), RADA(x=0.35, y=1) and CADA(x=0.2) show a gradual change with the spectral bands maintaining one to one correspondence with the paraelectric spectra, implying the glass transition rather than the structural phase transition. Temperature dependence of the Raman spectral change seems to show that condensation of the $B_2$ symmetry soft mode band, the intensity saturation of E mode $NH_4$ librational band and frequency changes of $\nu_2$ mode $PO_4/AsO_4$ internal vibrations are all correlated to each other with each one related to a different sapect of the freezing dynamics of the proton glass. Low temperature diectric peittivities RADPA(x=0.55, y=0.65) and CADA(x=0.2)mixed crystals also show the dielectric dispersion which is a typical behaviour of the dipolar glass freezing. For RADPA(x=0.61, y=0.65) mixed crystal, besides dielectric dispersion, some thermal hysteresis reminiscent of the phase transition with structural change is observed below $T_g$ in the real part of dielectric permittivity. At present, however, it is not clear whether the thermal hysteresis is due to structural phase transition or local symmetry breaking frequently observed in mixed crystals.
