Hi

I was wondering what spell schools I should get each of my mages to specialise in for max synergy/combo effect. Thanks!

(P.S. Planning on a party composed of 2 mages, a ranger and a tanky fighter.)

You should have enough points to get all spells anyway with two mages.
Anyway, couples are water+aero and earth+fire.
Of course water and fire are contradictory, but that's really not a big deal anyway.

water+fire, earth+aero is probably the most optimal.

Generally better, assuming you aren't stunlocked, to split the combo effects between the two characters so you don't have to wait a turn to create a combo.

If you're going to have a bajillion AP each turn though, maybe it's better to have geo + fire on one guy.

You only have so many points to put into skills so there isn't much point having them all with all 5 schools at a high level, better to put those points into other things/defences.

If you have both characters with witch you can double up on blind and lower willpower though.

Elemental affinity is a good pick for playing with two mages.

Even having a Bajillion AP, your starting AP is not that astounding until verrrrry late game.
So it is still a better idea to go (Hydro + Pyro) and (Aero + Geo).

Witch is always good either way. Elemental Affinity is amazing for Hydro, and more of a late game choice for Geo and Pyro. Never bother with Elemental Affinity for Aero.

In my very early game experience, I'd recommend having Pyro and aero on different heroes. This has allowed me to cast wildfire (haste) on my aero mage and then light the battlefield up with stuns. My pyro/geo/witch is definitely taking on a support role in the early game (level 7).

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