There are many different ways to achieve a bonsai Japanese maple.
They can sprout new shoots from older, bare wood so it is possible to let them grow really big to get a thick trunk quickly then cut back to a good 'bonsai' shape trunk and then grow new branches. Probably allow 10-20 years to achieve a good mid - larger sized bonsai this way. Pros: relatively quick. Cons: larger scars will take a few years to heal over, JM sometimes die back when pruned really hard, Some risk that fast grown trunks will end up as ugly stump bonsai.
Keep pinching and pruning: this slows the growth rate so probably more suitable for small - medium sized bonsai. Allow shoots to grow 20-50 cm then cut back to first leaves. New shoots will sprout and grow, repeat. Allow 5-10 years for a smaller bonsai up to 20-30 for medium sized or larger. Pros: smaller scars will heal quickly so few or no scars, You have much better control over the growth so chances of ugly thick stumps is reduced. Cons: growth is slow so allow more years to achieve good bonsai.
There are so many other slight modifications to both these and various techniques you can add to either or both to give better roots, better branches, etc. Always many variables.
Of course you might be quite happy to just have a couple of 'sticks in pot' bonsai. That can be achieved with very little effort in just a year or 2. you already have the sticks, just need to add pots next spring.
Please note that JM bonsai is not easy. Keeping them alive and healthy is usually quite straightforward however pruning techniques and follow up needs to be good or better as JM tend to get thickened ends of branches an/or ugly thickened places on the trunks if too many shoots are allowed to develop close together. I recommend trident maples for beginners as they are far hardier, grow quicker and a lot more forgiving of less than perfect pruning and management.
Good luck with your new babies.
There's not much you can do in the way of design at this stage unless you are aiming at a specific style or shape. You could wire and bend the trunks while they are thin and pliable. Later they will be far too stiff to bend much. Always make bends in very young trunks more exaggerated than you plan because trees tend to thicken more on the insides of bends so trunks gradually 'straighten out' as they grow and thicken. Monitor wired JM carefully because the wire will make marks on the delicate bark in just a few months.
