I have an all-round LWGC Mage specced in every magic school. Gets shit done and feels good. High-lvl spells included. Never regretted the decision to spec her so. 
Ditto.  My caster currently has 5 in two magic schools and 3 in all the rest, without neglecting defensive skills.
I really don't feel like having two characters is gimping me in any way.  In fact, sometimes I wish I could play with one character.  Half the time, my melee fighter's standing around watching the fireworks.  I probably should have started raising her INT and giving her a magical school or two to play with.