First: Dual-wield is not a skill-tree
Second: So if you would want to level Scoundrel and pyro at the same pace, you would have to pay 2 every level because of switching the focus?
The problem is not, that you can learn several skill-tree. The problem is, you learn every skill from a tree just by putting in 1 point. You are also not limited about the amount of skills you are learning, memory only limits the amount of skill usable.
In D:OS you need to skill further to learn more skill of a tree and higher tier skills of a tree. Those limitations are totally missing now and the main reason for all this mess. To make this undone would be probably the easiest solution. If a skill like Adrenaline is overused you could just Tier it up, and it would afford more investment automatically.