The dielectric relaxation spectra of poly(ethyl acrylate)-i-poly(ethyl methacrylate) sequential interpenetrating polymer networks, IPNs, are presented. In addition, dynamic-mechanical data were used to study the compatibility of the networks as the main relaxation predominates on the secondary relaxation in mechanical spectroscopy. When the IPNs are highly cross-linked, both networks are forced to mix at the molecular level, and as a consequence, a single main dynamic mechanical relaxation appears. By contrast, when the cross-linking density is small, the individual relaxation behavior of the component networks can be observed. The poly(ethyl methacrylate) networks with low cross-linking density show the merging of the secondary beta dielectric relaxation and the main alpha relaxation with the same characteristics as the poly(ethyl methacrylate) chain polymer. This behavior changes, however, in the highly cross-linked network in which the influence of the main relaxation on the overall behavior is smaller, and it was not possible to separate the alpha and beta components in the merging region. The incorporation of the poly(ethyl acrylate) chains to the cooperative motions of the poly(ethyl methacrylate) network shifts the alpha and beta relaxations to a different extent, which induces changes in the shape of the spectrum in the merging zone.