Late winter is the FIRST time wisteria are pruned during the year. Subsequent pruning to control growth is done in the summer through the autumn as needed. Drastic structural pruning is done in the fall. Hard pruning into old wood induces backbudding, as well as flowering (if done correctly) All that pruning is necessary to maintain shape and induce blooms.
What you're asking, however, is not really possible with the plants you have for a number of reasons. First, They're simply too young. Wisteria, unless grown from cuttings from a mature plant generally won't bloom until they are mature enough to do so. That takes about eight to ten (or longer) years.
Second, structurally, leaves and blooms on wisteria are a problem for bonsai wisteria. Leaves are compound and large. Blossoms are up to a foot long--there is no reason to grow wisteria other than the flowers. They are a rampant aggressive vine that puts out prodigious amounts of leaves and vines that make them a pain in the ass to keep up with in a pot. The trunks on your plants are physically not really big enough to support the growth needed to make a bonsai out of them. Growth is needed to thicken their trunks.
Third, pruning that growth will significantly slow the needed trunk thickening you need. There is no real way to prune them now to induce believable taper in the trunks of your plants without compromising their structural ability to support leaves and growth.
Fourth, smaller containers also restrict growth and thickening-although slowing growth on a wisteria is a relative thing.
Old wisteria bonsai are typically begun with large or even huge heavily reduced trunks dug up from gardens. Those trunks produce growth that is developed into branching over time. Huge trunks are necessary not only to produce flowers, but to support the heavy growth this species produces.
Wisteria is a problematic plant to use for bonsai. It is not a tree. It is a vine. It acts like a vine--with heavy top growth and very little thickening at the base. The plant devotes most of its energy to growing UP a stationary support structures like other trees. It doesn't produce strong wood and relies on support from other things it grows on. It has very aggressive roots and vines. Here in the mid-Atlantic states in the U.S., wisteria has become an invasive. It sprouts everywhere. If it is planted too close to houses, it grows so aggressively it can shift entire buildings off of foundations and pull siding off of buildings. In a container, it does the same, although if you put an air gap under the pot, roots won't escape into the ground. That rampant growth has to be cut back or limited because it can take over other bonsai near it--you can tie extending tendrils and new vines into loose knots to slow it down until pruning time.
All that said, if you want to prune your plants, hack away at them any time between May and August. They'll be fine. Good luck!
