Your 2-6 volt batteries in series are the same as a 12 volt battery for wiring purposes. If you are adding a "dual battery" system, you have a box with 3 posts that has diodes inside. Diodes allow current
to travel in one direction only. They keep two battery banks separate while allowing them to be charged by one alternator
. The center post (usually) goes to the alternator and the other two posts each go to the positive post of the different banks.
Some people just have a manual switch they turn on when the engine
is running. A dual battery charging kit cost $100+. You can make you own by buying
diodes for about $20.