Battery to battery charging - Ah vs Wh
You can fully charge battery B if you use a dc-dc converter (switching regulator) to drop the 12V to 5V. In this case the WH is the number to look at. You will have to take into account the fact that the batteries are not perfect and so you lose a bit of power in charge/discharge and the efficiency of the switching regulator (typicaly 90%+).

Think compressed air not water.

In a further attempt to clarify this say I have a fully charged 12V battery of 100AH made up of 6 cells, how many AH can I get from this at 6V? If I split my battery into 2 6V batteries each of 3 cells and 100AH and then connect them in parallel I end up with a 6V 200Ah battery.