my understanding of using multiple weapons is they really went for a simple method
your "main hand"makes all bab attacks, and iderative (sp)
so as a 1st-5th level fighter for instance you get 1 mainhand attack
at 6th level (+6 bab) you get 2 "main attacks" 1 at +6 1 at +1
at 11th level you get 3 "main attacks" 1 at 11, 1 at 6, and 1 at 1
etc.
if you are dual wielding, using a double weapon etc you always get 1 offhand attack per additional "hand"
so lets say you are using a longsword and shortsword/dager or similar
your mainhand can get multiple attacks but your "offhand" only ever gets 1 additional attack
this is where some of the "monster" races get nasty
dragon? claw, claw claw, claw claw bite, claw claw bite wing attacks etc
demons/devils naga etc with extra arms?
http://images.wikia.com/wowwiki/images/1/19/Shivarra.jpg is an example
this would have 1 primary attack (set) and up to 5 secondary offhand attacks.
as far as casting is concerned in general you will have 1 spell progression table
such as mage, cleric, paladin, ranger etc.
each time you level up in your casting class you advance 1 row (level) down the table unless otherwise dictated by your class