When you put two batteries in parallel they will share the charge among themselves.
For example, if you connect two batteries together, one fully charged and one dead the resultant combination gives you roughly two 50% charged batteries. This is in theory of course because the fully charged battery will dump a hell of a current into the dead battery initially. This may not be safe. Connecting two batteries of nearly the same charge level together is fine.
The same then holds true for charging. While charging two batteries in parallel, the batteries will have the same voltage and charge at the same time. The batteries will reach full charge at the same time as well.
The problem is that if one battery takes 1 hour to charge. Then two batteries will take 2 hours to charge and 3 batteries will take 3 hours to charge and so on. What every you think you gain by paralleling them together is lost do to time spent charging.