I know some trucks have dual batteries,like the diesel. But my gas job is like most and does not. But I like to !!! I know I can,and then hook them up. Like Positive-negative-postive-negitive. But is there a better way ???

 

Not quite sure what your after here. The only realy purpose for having 2 bats in a gasoline system is if you need it for short periods of high current operation while the engine is running(current that the existing bat and alternator cannot handle). Just not good to overdrive alternators, high heat = short life.
Or,if you need it because you are planning to use battery power while the engine is off, be careful. This can shorten the life of your batteries. The key word here being "batteries" because when it comes time to replace, you'l be spending $ for 2 bats, not just one. You'll be able to go a longer time between charges, but if you leave something on to long you'll ruin 2. If you do go with 2 bats, never let the battery voltage drop below 11.5 volts(w/engine off). If this happens, sulfate crystals will build up on the lead plates in the battery effectively lowering the total voltage the battery can charge up to. Many recommend not letting the battery drop below apx 11.7 VDC.
You described connecting the batteries in series. DON'T DO THIS WITH 12V BATTERIES!! This will put 24V + across your systems components and fry everything. You can only do this if you find some 6V batteries.
You will need to connect the 2 12V bats in parrallel(black to black and red to red). Do not mix old batteries with new ones. Alwayse install and scrap batteries in tandum. Do not mix brands either.
Also, when comparing batteries of different brands for value, here is something to keep in mind. . .When bats are of the same physical size, the heavier bat is the better one.
Sorry for being so long winded.

 

Why do diesel's need two batteries? Is it because they often idle for a long period of time under the optimum RPM for the alternator?

 

Could it be they turn over faster in the cold climatesand also operate glow plug circuits? Could it be the 20-1 commpression ratio? I think they need 2 batteries. TJ