Just an aside, you can do more than 1 spell per turn... there are consequences but the mage if he really wants can throw four F6 Stun bolts.
The difference is with each successive casting it gets +1 Drain Value. That and you split your dice pool 4 ways. So If you have Spell casting 16 Dice then each roll only gets 4 dice with an average chance of a hit being 1.33 which is easy to resist.... so most people don't cast more then 2 because the dice pool gets very low especially with moddifers.
So first stun bolt is F6 / 2 = 3 DV*
Second F6 / 2 = 3 +1 DV*
Third F6 / 2 = 3 +2 DV*
Fourth F6 / 2 = 3 + 3 DV*
These drain value's however are resisted each time with the FULL resist drain pool. So the first few castings if you have 9 or so you'll only be taking on average 6S from all of this... While potentially hitting someone 4 times with about 7S (if they don't resist anything)
Extreme sckewed example but i'm at work here so xD sorry! Just thought I'd make you aware of that.
*Disclaimer = Not the real drain value just an example to show how you can do multiple castings.
Also Mages have good ways to give people more Initiative passes for those who do not have IP's of 1 or higher. If they get a few foci they can bind them to the foci to give other team members on the job improved IP's and improved Reaction. 
Mage = teh awesome. xD
actually it says in the Core that you only need to split you dice pool while casting spells in the same action phase would that applied to casting also during IPs?
If so that would also apply to shooting and melee wich would make a whole lot sense.
I mean like i said to a player who wanted three fire bolts in the same turns during IPs "so let me get this straigh you think you could muster the power of magic, a dicipline that requires concentration while acelerated and performing the appropiated spellcasting for three spells in 1 second each? i think not"
I think you may have miss understood.....
This example is taking into account the mage's first initiative pass.....
So i'll do an example of what a mage like mine might do.
Say two mooks come out of a door and are here to get me. I win initiative and I have 2 IP's and they have 1 IP.
Well I definately want to take at least one of them out figuring these ganger mooks have low will power and assuming I don't have Stun Ball and only Stun bolt I'm going to try and knock them out.
I don't want to overcast and take a good amount of drain this early in the adventure so I'm going to cast low.... I have some odd 16 dice to spell casting so I decided I'll make sure to take one out, hoping I can take whatever slugs the other one throws at me or that i can dodge it.
So I tell the DM I'm using my complex action to spell cast, and I'm going to do two F6 stun balls. In order to cast 2 spells, I have to split my 16 dice of spell casting. So I decide each spell will have 8. I get about 2 hits on both, and the mook only has 3 will power. Average of one so each Stun ball takes it's toll of 7S and 7S. Dropping that first guy out of commision.
Resist drain with a low force is an easy feat, for stun bolt I THINK the real code is 6F / 2 = 3 - 1 = 2 DV so I really probably don't even need to roll but I will for DM's sake of getting no hits. With 11 or so dice it's easily made. Now for the second spell I cast in the same round it's 6F / 2 = 3 - 1 = 2DV +1 for being an additional spell = 3DV easily made again but if I had done another one.... it'd be +2. Ect.
Mook 1 is out cold, and mook 2 gets his turn he fires at me possibly does some damage (hopefully I got a good reaction / armor to cancel out most of the damage and hopefully it's physical so I can just HEAL spell it later).
Next is IP 2 oh look I have IP 2 and mook doesn't. So I'm going to do the same thing since it was so effective on the last one. IP 2 I spell cast and cast 2 F6 stun balls splitting dice evenly between. Same result, that the mook goes down to stunbolts to the chest. I haven't killed them so I now have the choice whether I need to kill them or if I can just leave him there to wake up with a really bad headache.
As you can see in 1 Combat turn I casted 4 spells with only 2 IPs. Now most people won't split their dice pools to cast mulitple spells but if you have enough dice it's quite possible to do. If i had kept all my dice in 1 Stun bolt I would have gotten about 11S compared to 14S from two bolts just going by averages, but it would have been harder to resist. That mook either one could have gotten lucky and rolled all hits with my average of 2 not being high enough to affect him.
But yeah I just wanted to let you know you can cast multiple spells in one IP it just ADD's to the Drain....