If you need 12 volts, you have 12 volts. Essentially, you have an old GM truck with a 100 amp 12 volt alternator to run 12 volt stuff to your heart's content. AND you have another 12 volt alternator, for extra stuff. You CAN get 12 volts from alt2 using the original wiring system, just use +12 as (-) and +24 as (+). THIS IS DANGEROUS ON NEGATIVE GROUND EQUIPMENT UNLESS IT IS ISOLATED PROPERLY. My second alternator does nothing after the battery recovers from starting my engine, so I have an extra 100 amps of free power over a normal Chevy truck. I have ALOT of isolated ground lights, heated seats, and auxiliary electric heater (instant cabin heat in winter, no waiting for coolant to warm up) and a 12 volt inverter mounted on pieces of ceramic (to isolate from vehicle ground) that all runs off my "extra" alternator, PLUS a TON more stuff that runs of the normal 12 volt side of the system. A new 24 volt alternator is like $100 on ebay. I take excellent care of my batteries, so I never have to worry about one dying, etc. Sure, if you had a 12 volt starting system, you could receive a boost from a civvy vehicle.. but you'd probably almost kill the civvy vehicle with such huge power demands on it's battery and alts. In my truck, I have no trouble starting in cold temps with 24 volts. My friends with 12volt diesels can't say the same. I go around boosting everybody off my batt2. Just don't let the vehicles touch and connect both cables to batt2 and don't touch the body with them. There you go, another handy use for your second 12 volt system, boosting friends without worrying about discharging your main battery, with no danger of voltage spikes frying your radio.
When it comes to high power demand, more voltage = more better. That's why cross-country power lines run like 230,000 volts. Ohms law shows half the resistance (lost power) in 24 volt system compared to 12 volt. That's half the power lost in the wire, worn solenoid terminals, even the brushes of the starter itself. (power losses you can't even measure unless you take apart a spinning starter) Not to mention how more voltage performs better in electromagnetic electric motors..
As was said, your truck, your call. I just want to make it clear to others considering a 12 volt conversion - you are no further ahead, and if cold weather is a factor - 24 volt is MUCH MUCH BETTER.
Even if you want a compressor under your hood.. I could do that with 2 alts no problem. Just put the compressor / generator / etc. in the empty void above the steering stuff on the driver's side, and run a jackshaft to the front of the engine with a pulley to a slightly longer alt1 belt.
I love my 24 volt truck. Wouldn't change it. Just my
.