The dice cap is actually Double your
natural Attribute+Skill rating, or 20, whichever is higher (p.62,
SR4A). Which means for a human, it's possible to get 24 or even 28 dice on a test, if they had maxed attribute and skill ratings and the Improved Attribute and Skill qualities. (I emphasized Natural because a lot of folks miss that. This does mean that augmented ratings do not count... So a Sammy with 6 Pistols, 5 Agility, and Muscle Replacement 4 would cap at 22 (Agility 5 + Pistols 6), not 30 (Agility 5 (9) + Pistols 6).
The other thing to not is that this cap is
after dice penalties. So if your natural cap would be 20, and your mods would give you a theoretical dice pool of 24, and you suffer 4 points of negative dice pool mods due to wounds or whatever, you still get your 20 Dice Pool, not 16. Negative DP penalties come off of Bonus Dice and Augmentations first is a good way to look at it.
Now, that said, looking at "Attacker using a Second Firearm" (p.150,
SR4A).
Split the dice pool before applying modifiers.
The interesting thing about that is when calculating max dice pool, you figure it out after penalties. Since you split your dice before that, in this instance I'd say that you split your full base dice pool (Skill, plus augmented attribute, plus specialization). You do not gain the benefit of a smartlink or laser site, and you apply all modifiers to each split pool (So negative penalties do kinda double whammy you). The only restriction here is that if you somehow had enough bonus and augmented dice to somehow push either of the split pools up over what your normal dice cap would be, you're still effected by the cap. Not certain how you could do that normally, but I'm sure someone could work that out.
I should note that the intention for splitting DP is to split the pool as evenly as possible, but... It doesn't say that you have to do it that way. So that might be one way to try and cheese the Dice Cap would be if your Natural Cap is 20, but you have 28 theoretical Dice Pool, and you split your dice 27 & 1. In this case, you still cap out at 20 for the first gun, so the best you could do is 20 for the first and 7 for the second, before modifiers come into play.
Bull