If they are wired in parallel they are both charged at the same time. Basically the two batteries act like one larger battery.
Yes if they are connected in parallel but I would not recommend it. Just charge them one at a time.
With the batteries in series, the alternator provides the same current, and therefore the same charge to all the batteries. The alternator can take no account of any current being drawn from the 12V take off point. Part of the charge current is being fed to the 12V load and not to the batteries. Therefore, the 12-24V batteries are being fully charged while the 0-12V batteries are being partially charged. Over a period of time, the difference between the charge levels will become more obvious.
They were both free of charge.
The two batteries can be wired in series BUT unless they are identical batteries in an identical state of charge they should be checked regularly to see if one of them is fully charged (the voltage has risen to 14.5 volts).
They were both free of charge.
No you cannot.
You need to multiply the number of coulombs by the number of volts. If the two batteries are in series, then you can add the voltage of both batteries.
It depends how they are connected. If the terminals of two 12 volt batteries batteries are connected to the same circuit (in parallel) then there is no change in the flow of current- it will just last for twice as long a time. But if two opposing terminals of the batteries are connected to each other first and the free terminals then connected to the circuit (in series) then there would be a total voltage of 24 volts and twice as much current would flow. The two batteries would last for the same length of time as one battery on its own.
Use a special charger, such as an RV or boat charger, that has multiple outputs (best). Connect the two batteries in series, and use a 24V charger (ok, if the two batts are exactly the same). Connect the two batteries in parallel and use a standard 12V charger ( worst choice, one battery tends to hog the current, and they receive unequal charges). See the links for a dual-output charger I Googled.
24 Volts.
step up trasnsformer <<>> As suggested above transformers do not work on DC installations. You will need to use two 12 VDC batteries in series with each other to obtain the 24 VDC needed to operate the motor. Connect a jumper between the two batteries with a wire the same size as the wire coming from the motor. The jumper will go from the negative of one battery to the positive of the second battery. The two remaining posts will connect to the motor and have a potential of 24 VDC.