Millimeter and sub-millimeter observations of continuum linear dust polarization provide insight into dust grain growth in protoplanetary disks, which are the progenitors of planetary systems. We present the results of the first survey of dust polarization in protoplanetary disks at 870 μm and 3 mm. We find that protoplanetary disks in the same molecular cloud at similar evolutionary stages can exhibit different correlations between observing wavelength and polarization morphology and fraction. We explore possible origins for these differences in polarization, including differences in dust populations. In contrast with the predictions of earlier models of dust polarization from self-scattering, which predicted a steep dependence of polarization fraction on observing wavelength, we find scattering polarization fractions of up to a few percent in RY Tau and MWC 480 at these two widely-separated wavelengths. For RY Tau and MWC 480, we present models of the scattering polarization from several dust grain size distributions. These models aim to reproduce two features of the observational results for these disks: (1) both disks have an observable degree of polarization at both wavelengths and (2) the polarization fraction is higher at 3 mm than at 870 μm in the centers of the disks. For both disks, these features can be reproduced by a power-law distribution of dust spherical dust grains with a maximum size of 200 $\mu$m and high optical depth. In MWC 480, we can also reproduce features (1) and (2) with a model containing large grains ($a_{max}$ = 490 $\mu$m ) near the disk midplane and small grains ($a_{max}$ = 140 $\mu$m) above and below the midplane.