Sec. Macrantha maples like this don't do well being cut into. Leave it to fork on its own.

Thank you, Embothrium!

Additional details can be found on page 136 of Brown/Kirkham, The Pruning of Trees, Shrubs and Conifers - Second Edition (2004, Timber Press, Portland/Cambridge).

I´m afraid I have no access to that book.

Other than buying one on the web a way to see it would be to look for it in the library of a college that has a horticulture program.

Prune to provide a structure to support the eventual canopy rather than make the tree look like a big one even though it's small.