Thank you for sharing this photo! If you have any other photos from Japan you feel like sharing - please do
I think the underlying problem with a slingshot tree is that you have two competing focal points. The only solution is to make one of the trunks a dominant focal point while reducing the other. This can be accomplished visually by jinning one of the trunks, or physically by cutting, reducing, or drastically pruning one of the trunks.
It is hard to see all the detail in your photo, but notice how the two slingshot trunks do not compete in the design. The larger trunk was jinned and dominates the design. The smaller live trunk is actually quite a bit smaller, and it is tucked into and balanced with the lines of the larger trunk. In my eye I don't see two competing trunks - just one cohesive design.
Also note that the angle of separation between the two trunks is narrow. The wider the angle, the more difficult it is to balance the design. In the case of my recently posted slingshot juniper, the two trunks separated at 90 degrees forming a very heavy and awkward trunk line that was more like a "L" than a "V". Even if I were to jin one of the trunks, I would still be left with the awkward intersection that would be impossible to mask.
One other possible solution that I considered is to create a new trunk from the live vein of bark on one of the sides. If you start thinking about that approach, suddenly you have a lot of options available. Even so, I'd be working to reduce the angle between the two trunks, and making sure that one of the trunks was considerably thicker and more dominant than the other.