Don't know how I missed a 2 trunk maple thread...
I like this tree, and how the trunks are to each other. I have a hackberry that has some of that feel. I agree that the secondary trunk needs to be cut lower. The one thing that I think is a miss on this so far is the pot that you have this in. It's too small to be doing the development that you are currently doing. If you put it in a larger pot, but a shallow one you will get more development faster. An overlarge pot will give you more growth, and what I've found is that if you develop JM in shallow pots, they tend to grow a better surface root structure. Of course screwing it to a board is the best option for that, and a training box or very large pot. I've found with smaller projects, going into a very shallow but wide pot does the same basic thing.