If you have more 1/0 I would add 2 more runs back (and make sure you double up on ground leads as well). Running dual 160a alternators, even assuming both of them are putting out the full 160 and it's getting to the back for the amps, you are still pretty shy of what a pair of 3500.1s are wanting. That's why you are seeing your voltage dip down as low as 12.4.
That voltage drop means you're not getting near the rated output of the amps and worse, when the voltage dips into the 12 volt range, you are likely driving the amps into clip (which is why it might seem like they are pushing the subs too hard despite their RMS rating, driving subs with a clipped signal will toast the subs, even if it's significantly lower power than they are rated for).
A lot of guys here are converting over to Lithium batteries. The Lithiums can maintain a higher voltage and consistent amperage draw for many minutes before having to turn down the system and allowing the alternator(s) to recharge them. They recharge faster than wet cell/AGM batteries as well. The drawback of course being that they are expensive.
Being only a week and a half out to your competition, unless you've got a bunch of money burning a whole in your pocket, you are probably stuck adding additional runs and making sure you've done your big 3/big six runs under the hood.
Past that, definitely look into some bigger alternators, those 3500.1s are going to need 350 amps each to reach their full potential. Even a pair of 300/320 amp alternators and plenty of OFC runs from the engine bay to the battery bank next to the amps will stiffen up your power and more than likely get you a few/several more dBs without changing anything else.
Hope this helps!