Yeah, this build would be a good buff monkey if extended spell was in use, but outside of that, the class mix would be really hard to pull off in NWN. In PnP it works fine because the caster-levels stack, so even though you don't get access to higher level spells faster, at least your lower level spells and DCs can max out.
Caster Level doesn't stack - it's your effective casting level for a given casting class.
Perhaps we did this as a house rule, but I do distinctly recall that we played 3rd ed. and all similar editions with the caster level total being gained from any level in which you had a caster class. So a 5th lvl wizard/4th lvl cleric/4th lvl fighter had a caster level of nine, because he had nine levels in a caster class, regardless of whether its divine or arcane or simply a different class. This makes more sense to me than having it limited by class, effectively making the character have two (or more) sets of caster levels. But anywho, I suppose now we've high-jacked the thread to discuss a PnP element that simply can't be applied to the NWN game engine regardless, heh.
This is the reason the Practiced Spellcaster feat exists... And I've seen implementation on NWN before.
Eh, familiar's aren't really meant as combat additions in Layo anyway. That's what spell summons are for. Familiars are there for RP purposes, like spying, scouting, helping with a RPd spell during a quest, relaying messages, acting as a local alert system, etc, etc. This being the case, having two familiars would actually be very beneficial in terms of RP versatility.
Just so you are clear, Practiced Spellcaster is not available as a feat in NWN.
So I'm assuming its one of the few (or many?) new feats added for layo.