Page 164
Multiple Attacks
A character may use a Free Action to attack multiple targets
in a single action (see Multiple Attacks, p. 196) by
splitting their dice pool. This action must be combined
with a Fire Weapon Action, Throw Weapon Action, Melee
Attack Action, Reckless Spellcasting, or Cast Spell Action.
You may not take multiple attack ACTIONS in the same ACTION PHASE.
So, if a mage uses the Multiple Attacks Free Action and the Cast Spell Complex Action, and splits his dice pool, he can cast two fireballs at two different targets - the same way that a samurai can do the same thing with a gun.
If a mage chooses to use the Reckless Spellcasting Simple Action twice in one turn, then only ONE of those spells can be an attack. Note that the spirit of that rule is that it doesn't have to be a "Combat" spell - so using Fling to throw a grenade at someone, or even using a Decrease Attribute spell would be considered an "attack".
That last part is VERY loosely defined, and is going to have to be up to you and your GM - I believe the "official" quote is "If it is something that your character would consider an attack on him if an enemy mage did it to him, then you can't do two of them in the same Action Phase."
Next up, about applying the "modifiers" before or after splitting dice pools.
Foci and Mentor Spirits are not "modifiers".
Mentor Spirits "add +2 dice to tests"
Spellcasting Foci "add their force to your Spellcasting dice pool"
Specialization "adds 2 dice to any test made for that
skill when the specialization comes into play."
category as the focus."
Modifiers refer specifically to "Ranged Attack Modifiers" - i.e environmental, situational, and wound (page 173) - obviously recoil modifiers would be included, but since spells don't incur recoil, this would be zero.
So your character would get 16 dice, divided by 5, and then subtract those modifiers.
If you character had the same 16 dice and shot someone, they would subtract those modifiers and THEN divide by 5.
Why they make that distinction I do not know, if you want my opinion, for what it is worth, it should be the same, regardless, since I can fire two guns at two different targets, which means that they could be at two different ranges, and so the modifiers would be different - but that is beside the point. Personally, in my games I make Multiple Attacks, regardless of the source split dice pools and subtract modifiers the same.