My house bank is only a bit larger than yours at 1350 AH, also made of 6v AGM's in series parallel.
I charge with a Penntex (large frame) 200A alternator - using a Balmar
ARS-5 programmable regulator. - I purposely dumb down the alternator(via the regulator) to 175A max, which keeps things running plenty cool.
is a Cummins 5.9 with a serpentine belt which provides plenty of belt to run the alternator - don't know how your Perkins is set up, but anything over 110A will require 2 belts or a serpentine - you may also have to consider side loading bearings on water
pumps etc. if using old style belts.
As my boat is a single
, I use a Digital duo (echo charge) unit for recharging the grp 31 start battery.
In your case, why not put a large frame alt(ebay $200) on one of your motors with a good programable regulator - (which you will need to give those AGM's the slightly elevated voltage they need to charge right - contact Fullrivers engineering rep for the exact specs for your batteries)
mine spec'd out to:
Leave the stock alt on your other engine to recharge the start battery - it does not need a 3 stage regulator.
my system works great for our use - I can anchor
up to 6 days without starting the motor
if desired and still have 60% SOC. Normally we are recharging at 80% SOC and can fully recharge in 4-5 hrs cruising including float.