the batteries are currently in serries to make them 24 volt, If you change them to a parallel circuit you will then only have 12 volts. If you have done the conversion and have no other devices that need 24 volt then yes, you can take both possative terminals and put them to the fire wall block. The rear battery will need the negative strapped to ground just like the front battery in order for it to ouput 12 volts currently there is no negative cable for the back battery.
A serries circuit is running one battery into the other and when this happens you add the voltage of all batteries together in the circuit to get the final voltage. 2-12 volt batteries would be 24v. 3-12v batteries would be 36 volts. A Parallel circuit is where you have seperate feeds from each battery that are connected to one power terminal. Thus the feeders are parallel and voltage does not add but the Amp hours do. (starting batteries are also rated in cranking amps, not the same as amp hours)
All batteries are rated in Amp hours. 1 amp draw for one hour = 1 amp hour at the specified voltage. Most car batteries are between 70 and 100 amp hours at 12 volts.
If you have 2 batteries in serries that are each 100 amp hours then you have a circuit that is good for 100 amp hours at 24V. If you have 2 100 amp hour batteries in parallel, then you have a circuit that is good for 200 amp hours at 12volts
The cranking amp rating comes from the size of the interanal parts of the batteries. The larger the cranking amps the larger the internal bus bars are that connect each cell inside the battery.
I hope this helps.
