Context. To address questions about the physical nature and origin of spiral arms in galaxies, it is necessary to measure their dynamical properties, such as the angular speed, Ωp, or the corotation radius. Observations suggest that galaxies may contain several independent spiral patterns simultaneously. It was shown that so-called non-linear resonance coupling plays an important role in such systems. Aims. We aim to identify cases of independent spiral patterns for galaxies with a flat rotation curve and to investigate what relative pattern velocities, Ωpout/Ωpin, they might have for all possible cases of coupling between the main resonances. Methods. We solved equations for the main resonance positions (1:1, 2:1, 4:1) and estimated the ratio ω of the corotation radii for two subsequent patterns. For six close galaxies with flat rotation curves, we collected the measurements of the corotation radii in the literature, using at least three different methods in each case for credibility. We found at least two independent spiral patterns for each galaxy and measured the ω ratios. Results. We found ω ratios for all possible cases for the main resonances. For three cases, we obtained ω > 3, indicating that it would be difficult to fit two or even more spiral patterns in the disc. These ratios have been used to derive the wind-up time for spirals, estimated to be several galactic rotations. We find that three pairs of coupling cases, including those that have been vastly acknowledged in galaxies, namely, OLRin = CRout & CRin = IUHRout, have very close ω ratios; hence, they ought to be found simultaneously, as observed. We find a strongly confirmed apparent resonance coupling for six galaxies and we show that the observed ω is in agreement with theory. In two of them, we identified a previously unreported form of simultaneous coupling, namely, OLRin = OUHRout & OUHRin = CRout. This result was also predicted from the proximity of ω.