I do not think that the elemental damage on wands gets a bonus from points into the associated element.
They changed Earth wands to Poison specifically because they do.
For example, if I have a character wielding a pair of fire wands, has 2 points in ""(+10% damage), one point in "aerotheurge"(+10% vs magic shields) and one point in (+5% damage on fire attacks), will her wand attacks against magic shields deal 125% of normal damage, or 127%?
It's multiplicative, but the bonuses from weapon style(dual-wielding) and elemental boosts(pyrokinetic) are already calculated beforehand for you and shown in the weapon damage tool-tip.
Essentially, it'll be 110% of the displayed value against shields.