  We present HST WFPC2 observations, supplemented by ground-based H-alpha data,
of the star-cluster populations in two pairs of interacting galaxies, selected
for being in very different kinds of encounters seen at different stages.
Dynamical information and n-body simulations provide the details of encounter
geometry, mass ratio, and timing. In NGC 5752/4, we see a weak encounter well
past closest approach, after about 2.5x10^8 years. The large spiral NGC 5754
has a normal population of disk clusters, while the companion NGC 5752 exhibits
a rich population of luminous clusters with a flatter luminosity function. The
strong, ongoing encounter in NGC 6621/2, seen about 10^8 years past closest
approach between roughly equal-mass galaxies, has produced a rich population of
luminous clusters, particularly young and bright in a small region between the
nuclei. This region is dynamically interesting, so strongly perturbed that the
rotation curve reverses sign. Cluster formation requires a threshold level of
perturbation, with stage of the interaction less important. The location of the
most active star formation in NGC 6621/2 draws attention to a possible role for
the Toomre stability threshold in shaping star formation in interacting
galaxies. The rich cluster populations in NGC 5752 and 6621 show that direct
contact between gas-rich galaxy disks is not a requirement to form luminous
clusters, and that they can be triggered by processes happening within a single
galaxy disk (albeit triggered by external perturbations). (Abridged)
