Just to clarify, the two separate alternators, 1 each 24 and 1 each add-on 12 are completely seperate electrical systems.
The 24 volt system would be the OEM system tied into all the OEM electrical components with the two 12 volts batteries below in the OEM battery compartment.
The add-on 12 volt alternator will need a 12 volt battery (battery #3) to take the charge and distribute 12 volt power from that 3rd battery.
All wiring including the 2nd master switch, fuses (or circuit breakers), etc. is separate and the 12 volt consumers should have seperate isolated grounds returning to the 3rd battery.
My unsolicited opinion is that your additional equipment that runs on 12 volts only would be best supported by the add-on 12 volt alternator.
There is a member here on S.S. that has a really neat 12 volt (approx 165 amp) GM based alternator that will go above the OEM 24 volt alternator or generator or above his 24 volt GM based alternator either way.
You will need larger diameter alternator belts to include the 12 volt add-on unit into the drive loop but that is the only engine modification other than the obvious addition of the actual 2nd alternator.
Two seperate isolated systems is what you will have that way but it is really easy to install if you take your time and be careful in your installation you will have years of uninterupted service from it..
Most people with the add-on 12 volt system install the 3rd battery under the passenger bench seat (or so I am told). It sounds to me in reading your thoughts on added equipment that you may be approaching 50 amps + in a continuous draw in the summer with A/C on.
As suggested by others, sit down, draw it out both ways and then pencil in the component costs and do what is best for your needs.
My thinking is that you will choose the add-on 12 volt system but it is your truck and your buget. Just make sure that you protect wires and cables running through the firewall, floor, and any other sheetmetal containers, etc.
Big cables can go to a dead short and start fires faster than you can get pulled over in certain driving conditions. SAFETY above all else can be had with either system as lone as one crosses their "T's" and dots their "I's" by protecting the wire harness.
You will be able to get all of your questions and concerns answered here on S.S. if you ask away. There are some very helpful guys and gals here to assist if needed.
The 12 volt add-on system IS in my future for my M35A2C but has not been passed by the bidget committee to date.
I would not tap off one of the OEM batteries for more than a cell phone charger as an imbalance can kill your very expensive military batteries. but that is my 1.7 cents worth.
I am curious what you do decide to install and would like to know how it comes out if you will share with the group.
Regards, jimm1009
PS: You can also use a select group of 110 volt appliances if you step up to an inverter or 12 volt DC appliances are avaialbe to some degree too in the RV community.