Can I get some advice / ruling on splitting the spellcasting pool.

Am I right in this?

Mage M5 SC6 Specialised for combat spells +2 Mentor Sprit Wise Warrior +2 R4 Spellcasting focus +4

Normal pool would be 19
if split between two targets (casting a mana bolt at two guards similtaniously)
G1 5 from base pool (5+6/2) +2 Specialised +2 Mentor +4 Plus focus Total 13
G2 6 from base pool (5+6/2) +2 Specialised +2 Mentor +4 Plus focus Total 14

Rather then just
G1 10
G2 9

Because the specialisations, mentor spirits and focuses are modifiers to the skill and are added after the base split

Or is the focus bonus split too? Its being used at the same time in the same complex action to do both spells but am I understanding this right. I was sure of it when I did it last session but now I'm having doubts

According to the wording of the split description in SR4A it would indeed be your 13/14 calculation. I dont know if there are elaborate rules on this in Streetmagic.

It has been argued that the specialisation is part of the skill and therefore should be included in the split, but by RAW you are correct as specialisations are defined as adding dice to tests rather than added points to the skill rating.

There is no mention of a penalty for multiple targets except where it states indirect combat spells (so not manabolt) are treated like ranged attacks. Literally taken the second indirect combat spell in a multicast should suffer the -2 penalty, but if that's a bit inconsistant for you then a handwave wouldn't be a big deal.

Foci are limited to one per dice roll but not in number of uses per action phase.

So yeah, you've got it right apart from mentioning the increased drain. It's +1 to each drain value per spell beyond the first.

There aren't.

You've got it right lansdren. Do keep in mind though that any penalties incurred (Bad lighting, sustained spells, injury) will also subtract from both pools.

Good point on that one although our GM has been very light (pun unintended) on those kind of penalties (both to me and the more normal gun bunnys)

And as previously mentioned the small increase in drain but seeing as the drain roll stays the same its almost crazy not to double cast sometimes. At the moment I'm at 13dp for drain (5+5) from stats plus 1 from quality and +2 from centering (with plans for a little logice boost at some point too)

Although granted I did double mana tap someone and put them far into physical from overflow and pretty much have his brain coming out of his ears but for a silent take down it has its place

Although further thought would say double tapping would make tracing the astral sig abit easier even at low levels I think (Fluff yes not so sure on the crunch right now)

Before someone quotes the FAQ, this is one of the instances where the FAQ is definitely wrong but arguably preferable. Indeed I prefer it, but mostly due to my particular style of bookkeeping.

It's not any easier to track. There are, however, two signatures - one from each spell.

Meaning if you don't want to risk being tracked from it, you have to spend twice as long erasing the signatures (or use Cleansing).

Are there any other instances where a split like this works, or is it limited to spellcasting and the relevant bonus mods available. I know smartlink doesn't apply when shooting 2 guns, but are there any other mods from ranged combat that open the door to splitting skill/attribute pool and adding bonuses to both?

Specialty works according to some... reflex recorder would not work, adept powers would not work... lots of penalties from things like bad vision would mess up both pools... Oh, personalized grip works on melee weapons.

That's honestly all I can think of... Oh, tacnet would work as well.

Ah so...that's an excellent one to bring up...helps balance out non-casters...

"Certain Augmentations" is not clear.

"Dicepool Modifier" is a game term with a given definition (pg. 61 of The Precious), Specialisations (pg. 84) are a skill test modifier. Modifiers after splitting the pool is explicit for Firearm attacks (pg. 150), and implied for multicasting (pg. 183).

Are split dicepools not sufficiently ineffective? If you don´t think so, would it be enough to calculate the final dp, then split those dice?