Unless I am missing something, I don't see a second leader.....unless you mean that low branch off to the right. I wouldn't consider that a "leader" and at any rate, Japanese maples do not really have distinct leaders unless specifically trained to, as some weeping forms are. Once they start maturing and branching well, they seldom develop a 'leader' other than whatever trunk structure they may have. Removing anything that may be a considered a leader at this young age is unnecessary unless it has developed an uncomfortably tight crotch angle. If it is that lower branch you are referring to, removal is up to you. Personally I'd let it go for awhile to see how it develops - depending on your specific aesthetic, low branching on JM's is often considered a desirable trait.

As to the ID, that leaf is very heavily dissected yet good sized. I'd guess Acer japonicum 'Aconitifolium' or the fernleaf fullmoon maple, an extremely desirable selection :-) What does the fall color look like? If Aconitifolium, it will be on the smallish end of the scale, growing slowly to about 15', give or take. And Japanese maples do not develop invasive root systems - your retaining wall should be fine.

GG

I don't think that leaf would properly be described as dissected. That pic shows a typical matsumurae leaf form. It's deeply divided and serrated it looks like 'Sumi nagashi' but 5 lobed and green.

The form of that plant looks a little like A. japonicum.
I would not prune it at this time. Maybe after leaves fall you could thoughtfully consider shaping it incrementally.

Without question, the lower leader needs to be removed. Branching that close to the ground is unacceptable. You can also see by the growth rate of that branch and the way that it's trying to arc up that it wants to be a co-dominate leader. It's true that Japanese maples don't always need the strict pruning practices of a large tree. You can leave branches on them that sort of give them a large bonsai appearance. However, this branch ruins the aesthetics of the tree and makes it quite ugly.

Branching that close to the ground is unacceptable.

Based on that premise, most of the J. maples at the last two nurseries I've worked (both JM feature nurseries) are "unacceptable". Low branching on many forms of JM is extraordinarily common and in many cases very desirable and to be expected. It's important to keep in mind that many cultivars of JM are much more shrubby in form than tree-like and making broad generalizations about where it is appropriate for them to begin branching is irrelevant.

I'd agree with herman that a 'wait and see how it develops' attitude makes the most sense at this time. You may decide to remove it later, but since Japanese maples do not develop clearly defined leaders, this branch poses no threat to the health or eventual development of the tree. And herman, I guess I used the term 'dissected' to refer to its defined meaning (deeply divided) as opposed to a specific classification of maple. I also agree we are probably looking at an Acer japonicum :-)

Wait until late fall/winter to prune. Pruning is largely the choice of an individual as these trees, unless crossing branches or two twiggy, grow just fine themselves often.

And I'm leaning HARD to Acer palmatum 'Omure yama'

Here is a link that might be useful: Omure yama

JMs are considered feminine trees and a low growing branch that is trained/treated as a secondary tree off the same trunk is a mother/daughter trunk arrangement (Sokan, in Japanese). The branch is low enough to work in this style if you want to begin pruning with that in mind. You should introduce some movement into the trunks (soft curves) if you want the arrangement to be believable (natural looking), and the daughter tree should have a thinner trunk & be from 1/2 - 2/3 the height of the mother tree. If you don't want to mess with it, as GG says, you can always remove it later.

Al

Thank you, everyone, for your thoughtful and helpful replies! I can't remember what the autumn color looks like, but I will definitely keep a close watch on it this year. I plan to hold off on the pruning for now and watch how it develops. I was worried that waiting to prune things off could cause a problem, but that doesn't sound like the case. As for identification, thank you for pointing me in the right direction!

GG,

I think your tree looks like Ap 'Elegans'. It might also be a seedling. As FF says the leaves do look like 'Omure yama' but the form is more upright.

Post fall colors for us please.