It looks quite small from the picture. I would let it grow several years before I start to worry pruning.

Normally I'd also say to wait to prune that little guy, and leave the dual leaders since most JMs end up having several leaders anyway.

But in the case of an 'oshu shidare', I might reconsider. Since it is a cascading type rather than an upright, it might indeed look better with a central leader up to about 4 ft or so.

So if you prefer a central leader up to a point before the limbs start to spread outward and droop (the umbrella look), it would probably be best to go ahead and prune it before this spring.

However I've seen pictures of oshu shidare that didn't appear to have a central leader, and they looked fine.

This message is a little old. And you might already
be aware of this by now. I have an old bloodgood JM.
Unlike our native maples, my JM doesn't have a terminal bud. It puts out two laterals. So, the branches always fork.
I've seen a few with clear trunks for some distance. I suspect they had one lateral pruned off each year for the
first few years.

Here is a link that might be useful: See entries under ID Features