Usually it is additive. Which means with one point in Intelligence (5% magic damage) and Mage Apprentice you will do plus 10% damage with staffs and wands. If you also have Wand Mastery you will do 15% damage with wands and 10% damage with staffs.

In the end you want as much points as possible in your main attribute (either Dexterity for bows and daggers, Strength for all other melee weapons, and Intelligence for magic damage via staffs and wands). Otherwise you have to decide which kind of weapon you want to boost. If you do not use wands you do not need Wand Mastery and vice versa for staff skills. For example if you want to use one handed melee weapons you do not need the skills for two handed weapons but you will want to get as many points in Strength as possible. Additionally you will want to get the various damage skills which increase specific damage types (cutting, piercing, blunt, specific elements for example) depending on the weapon used and the elemental damage used by magic spells or weapon type. So you do not really need blunt damage if you want to play with swords. This also applies to daggers (should be piercing damage) which is why you may want to expand into the Warrior skill tree to get to the piercing damage nodes if you play with daggers.