The phase transition in ferroelastic Pb(3)(PO(4))(2)-Pb(3)(AsO(4))(2) mixed crystals shows typical multiscaling behaviour with two relevant length and timescales. One length scale is macroscopic and shows uniform, weakly first-order phase transitions between a rhombohedral paraphase (R3m) and a monoclinic ferroelastic phase (C 2/c). The second length scale is on the level of tetrahedral complexes which display monoclinic distortions at temperatures well above the macroscopic transition point. For instance, in Pb(3)(P(0.43)As(0.57)O(4))(2) the AsO(4) polyhedra show static deformation up to ∼60 K above the deformation of the PO(4) tetrahedra. The two timescales are either short compared with the time of observation, namely the dynamic reorientation of the PO(4) tetrahedral distortion, or very long. The long timescale refers then to the (quasi)static distortion of the AsO(4) tetrahedra which persists at T>T(c). These distortions appear to be uncorrelated or only weakly correlated and their random field leads to an order/disorder aspect of the phase transition which remains, on a phonon timescale, essentially displacive in character.