  Transition disks have dust-depleted inner regions and may represent an
intermediate step of an on-going disk dispersal process, where planet formation
is probably in progress. Recent millimetre observations of transition disks
reveal radially and azimuthally asymmetric structures, where micron- and
millimetre-sized dust particles may not spatially coexist. These properties can
be the result of particle trapping and grain growth in pressure bumps
originating from the disk interaction with a planetary companion. The multiple
features observed in some transition disks, such as SR 21, suggest the presence
of more than one planet. We study the gas and dust distributions of a disk
hosting two massive planets as function of different disk and dust parameters.
Observational signatures, such as the spectral energy distribution,
sub-millimetre, and polarised images are simulated for the various parameters.
We confirm that planets can lead to particle trapping, although for a disk with
high viscosity ($\alpha_{\rm{turb}}=10^{-2}$), the planet should be more
massive than $5 M_{\rm{Jup}}$ and dust fragmentation should occur with low
efficiency ($v_{f}\sim30\rm{m s}^{-1}$). This will lead to a ring-like feature
as observed in transition disks in the millimetre. When trapping occurs, we
find that a smooth distribution of micron sized grains throughout the disk,
sometimes observed in scattered light, can only happen if the combination of
planet mass and turbulence is such that small grains are not fully filtered
out. A high disk viscosity ($\alpha_{\rm{turb}}=10^{-2}$) ensures a
replenishment of the cavity in micron-sized dust, while for lower viscosity
($\alpha_{\rm{turb}}=10^{-3}$), the planet mass is constrained to be less than
$5 M_{\rm{Jup}}$. In these cases, the gas distribution is likely to show
low-amplitude azimuthal asymmetries caused by disk eccentricity rather than by
long-lived vortices.
