Earth magic has many skills that can cause poison as well as a skill that puts oil on the ground. Those effects are great fun to use fire on. If your mage that wields Earth magic has higher initiative than the Fire mage you will be able to use many combos in the same turn.
Rain (Water magic) increases the chance to stun with Lightning spells (Air magic).
Puddles of water can also be targeted by Lightning to stun those standing in the puddles.
I'd recommend to give both 1 point in witchcraft to get access to Oath of Desecration. The damage buff helps spells as well as physical damage.
And on a rather unrelated note - the warrior skill Divine Light seems to remove hazards in the cone area. If you throw around a lot of poison and need to clear a path for your warrior to get in close that might be useful.
A list of skills if you wanna theorycraft some combos before using them in game:http://divinity.gamepedia.com/Skills