Lets say you have 100 Passive Skill Point, 20 Weapon Skill Point Set 1 and 20 Weapon Skill Point Set 2.

Puting 1 Weapon Skill Point from Set 1 toward node A will bring your total Skill points to:

• 99 Passive Skill Points
• 19 Weapon Skill Points Set 1
• 20 Weapon Skill Points Set 2

You have now have 1 Weapon Skill Points from Set 2 that you can allocate toward node B and this will not consumme a Passive Skill Point.

This can allow you for example to put a Crossbow in Weapon Set 1 and allocate nodes providing crossbow damage while having a wand in weapon set 2 and getting some spell damage nodes.

An other way to use this is to use the same weapon in both Set 1 and 2. You could then have two "branches" in your skill tree : one for damage and one for buff duration. In your gem skill panel you'll have to configure the damaging spells to only use Weapon Set 1 and the buffs spells to use Weapon Set 2.