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I'm not sure exactly what your asking..
First I need to establish what type of vehicle were dealing with? and 6 volt or 12 volt?
Next , I did a brief search..I found nothing that exists in a duel voltage battery, You may be confusing it for those that were 2~ 12 volt battery's in one (had 4 terminals) and if one half went dead, you could jump from the other half..Or maybe one with duel terminals (side and top post)
The old "SOFT TOP" 12 volt non maintenance free battery's had two flat lead terminals on the top , where if you tapped between them, you could get 6 volts..But I haven't seen one of these in years..
You can put 2 Six volt battery's in series to get 12 volts, but on a 6 volt car it has to be hooked up : one battery hooked to power and ground on the Vehicle, Then a Cable run from the positive of the first battery, to the Negative of the second battery, then a Power cable off that to a 12 volt buss...The down shot is, the #2 battery would never get a charge.
I don't think even an isolator will work because #2 battery must be separated from the vehicle system or else it will (can) produce 12 volts..you'd have to come up with a pretty complicated automated cable switching system to make it work.
The Danger in running a duel voltage system is in future you can make a mistake and accidentally grab a 12 volt wire and hook it into a 6 volt system, and fry things..including the harness..or your whole rod (older vehicles didn't have fuse links ) Charging is usually a hassle, and replacement parts must be thought out..(Gee, was that 6 volts or 12 volts?)
You can't charge 12 volts on a 6 volt system..The generator won't reach enough speed to do so, and If it did, the regulator won't allow it to go that high.You can do it the other way around, and regulate it for 6 volts...but this is whistling past the graveyard..If the regulator fails, you will overcharge the battery and possibly explode it.
If you feel you need 12 volts, the BEST thing to do is plan a harness upgrade to 12 volts..Older wire is cotton clad, soaks up 50 + years of oil, gas and grease..and will support a flame..AND act like a FUSE or WICK if ignited!
To upgrade your system, you need to change :
All the lighting
Alternator/generator swap out
Heater motor and Speed controller
Wiper motor and speed controller
You can run the stock instrumentation on a dropping resistor as well as a stock radio (if retro theme)
You can run a 6 volt motor at 125% of rated value, but you drastically shorten It's lifespan..so starters , heater and wiper motors should be upgraded. The Solenoid WILL NOT run on 12 volts..(besides, 6 volt stuff is getting hard to find anymore as a "SHELF ITEM" where 12 volt stuff that will fit the block are standard.
If your wipers are vacuum, It's a good time to upgrade to electric..at best consider a vacuum canister for continued operation going uphill..
Install fuse links if your new harness does not already have them, and I'd put them on your 6 volt system now while planning the change to avoid tragic fires.
That's about it..
There are good cheap harness makers out there , if you do a search or go to the thread at the top of this forum (comparing harness makers) and plan around one.