Dual alternators should include dual-diode isolator for one battery (two into one). Dual batteries should include dual-diode isolator
from single alternator. Dual alt w/dual batt should have both isolators or neither one to work best.
My big Ford has dual batts with factory isolator. My just-bought 2012 Sierra HD has never had hitch, harness, or plow (NO witness marks) but someone added a second battery without isolating it. Two connected in parallel this way is bad juju, as current 'loops' between non-isolated batteries can shorten their lives.
Some 'dynamos' (H-D, MerCruiser) are also referred to as 'brushless AC generators', with 'permag' outer rotors and current control by inner coils' module vs by field excitation.
Some old Triumph bikes had an alt/magneto setup they called 'Energy Transfer'. IIRC it used two coils/points for ignition and the other four with zener diode/heat sink for regulation and 'battery eliminator' capacitor to reduce headlight flicker at low rpm. The setup was scrapped for full bore Lucas crap by the era of 'unit construction'. Clattering Amal carbs didn't keep the company down all by themselves.
btw, when I bought my old JD 1520 'gas' it had been fitted with a 1-wire auto alternator that performed all that was expected. I'd consider that alternative to OEM any time cost and room to mount looked as good.
