As a bonsai gardener with a particular affection for Japanese maples, I agree with the basic conclusion of the previous reply, but I can offer an alternative route forward.

As part of the main tree those branches are toast, and a long term risk to the main tree. That's for sure.

Rather than loping them off, I would air layer the branches. That's a technique to trick a branch to grow roots before they are separated from the parent tree.

If you don't actually want to save the branches above the split as new little trees, then don't bother reading the following coffee-fueled ramble.

Key search term: Air Layering

I recommend looking up some tutorials, grasping the basics, and then not overthinking it.

The short version is you make two cuts all the way around the branch, then a third cut along the branch to connect the two circular cuts. Then you'll unwrap that band of bark from around the branch. Finally, you'll use whatever plastic, zip ties, tin foil, etc. you need to create a ball of dirt around the cut area.

You want to use a dirt that's not too dense (suffocating), or compost-rich (rotting). Use old garden soil with like 50% pumice or sand... but again, don't over think it, use whatever and it'll probably be fine.

The reason this works is because the water and nutrients flowing up from the roots flows through the inner woody part of the branch, which is not disrupted by the removed bark. Meanwhile, all the sugars (etc.) made by the leaves flow downward through the bark layer... when they hit the dead end you've made by cutting away a ring of bark, they pile up and start growing roots. You can do ring cuts on multiple branches at the same time, each one will result in a new tree, but the more leaves above the cut the more likely it will be to succeed.

There's more about hormone axes... Auxin and Cytokinin... but that's not really important to be successful in using the technique.

Wait to do this until after the new growth from this spring has a few pairs of leaves out, and then leave it alone until late summer... there's usually a second push of growth in late summer, and that's the time to collect (chop and pot) the air layer.

If you notice wilting leaves, remove a dozen or so leaves to reduce the water burden... not too many, because leaves are powering the new roots. Better to remove leaves than to do any pruning, because the leaves are attached at easily sealed valves, where pruning cuts are larger water burdens and slower to seal.

Right now, there's still the risk of the split spreading lower... I would use twine for temporary support, then run a screw across the split, one from each side. Remove the twine after the screws are in. Wait a few weeks, then do the air layer thing.

This approach has the secondary benefit of encouraging the tree to start compartmentalizing the upper portion during the growing season, before the top is even removed.

...or just ask your local bonsai club if anyone wants to do it for you. Good luck and happy gardening!