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.
