A digital twin Mach-Zehnder holographic interferometer is presented and applied to the investigation of fracture mechanisms in resin concrete submitted to a three-point flexural loading. The double interferometric setup allows spatial multiplexing for a simultaneous double sensitivity measurement. The component determination is obtained from two different illumination directions giving two sensitivity vectors. The numerical reconstruction of the object is performed following the discrete Fresnel transform. We investigate the image formation by merging analogic and discrete Fourier calculations in a smooth reference plane wave scheme. We examine the complex relation between the original object and the reconstructed one and the object localization is derived. Experimental results are presented and exploited to obtain the evolution of the crack tip propagation during the test. © 2004 American Institute of Physics.