The L10 ordered MPt(001) thin films (M = Fe or Co) are very interesting for perpendicular recording due to their magnetic anisotropy and magneto-optical behaviours. Epitaxial L10-ordered NiPt(001) / FePt(001) bi-layers were co-deposited on MgO(100) substrates by MBE. The L10 order parameter is high with the concentration modulation along the growth direction. Some FeNiPt2(001) thin films were obtained by interdiffusion of the bilayers. The long-range-order parameter is conserved after interdiffusion (S = 0.75 ± 0.06), which can be explained by different mechanisms: a second-neighbour jump, a six-jump cycle, an anti-structural bridge mechanism or an antisite-pair elimination and creation mechanism, a double vacancy or a triple defect diffusion mechanism. Quenched molecular dynamics calculations in the frame of the second moment approximation of the tight binding method have been performed to obtain the energetic paths of the different mechanisms. The secondneighbour vacancy jump, the simultaneous jumps of bivacancies and the triple defect mechanisms can be ruled out for energetic reasons.