Iru knows what he's talking about.
Disciplines? What are those?
Sorry spell schools
But summon spider is a Geo, wouldn't it be better to have +2 points in either Aero or Hydro form the get go?
It won't hurt
What you suggest seems like I have to spec in 4 different abilities, wouldn't that be wasteful if I've to max them all?
Nope, you'll have plenty of ability scores to use and you want as many party members as possible with as broad a skillset as possible. It sucks when only one guy has two spells you need to cast.
So "Far Out Man" for both and "Pat Pal" for one of them
Right
What about willpower and such?
Ignore them. Your defense is distance, crowd control, summons and Teleport.
Also, who would you suggest I give what to? Giving the same stuff to both mages would be silly, wouldn't it?
Nope, give everyone everything. You might have one guy who hits 5 in Hydro first while the other one hits 5 in Aero first, but in the end you want at least 4 schools at 5 on each character (and 3-4 in the 5th school).