  Context. The mechanisms governing the opening of cavities in transition disks
are not fully understood. Several processes have been proposed but their
occurrence rate is still unknown. Aims. We present spatially resolved
observations of two transition disks and aim at constraining their vertical and
radial structure using multiwavelength observations. Methods. We have obtained
near-IR scattered light observations with VLT/SPHERE of the transition disks
J1608 and J1852. We complement our datasets with ALMA observations and with
unresolved photometric observations covering a wide range of wavelengths. We
performed radiative transfer modeling to analyze the morphology of the disks
and compare the results with a sample of 20 other transition disks observed
with both SPHERE and ALMA. Results. The scattered light image of J1608 reveals
a very inclined disk, with two bright lobes and a large cavity. J1852 shows an
inner ring extending beyond the coronagraphic radius up to 15au, a gap and a
second ring at 42au. Our radiative transfer model of J1608 indicates that the
millimeter-sized grains are less extended vertically and radially than the
micron-sized grains, indicating advanced settling and radial drift. We find
good agreement with the observations of J1852 with a similar model, but due to
the low inclination of the system, the model remains partly degenerate. The
analysis of 22 transition disks shows that, in general, the cavities observed
in scattered light are smaller than the ones detected at millimeter
wavelengths. Conclusions. The analysis of a sample of transition disks
indicates that the small grains can flow inward of the region where millimeter
grains are trapped. While 15 out of the 22 cavities in our sample could be
explained by a planet of less than 13 Jupiter masses, the others either require
the presence of a more massive companion or of several low-mass planets.
