1] VOLTAGE: 3.6V or 3.7V - 18650 Li Ion Batteries
All single cell lithium ion batteries are going to be 3.6-3.7v. There are applications where multiple cells will be tied together in series. This will result in voltages that are multiples of 3.6-3.7v. So as long as you match the number of cells and approximate mAH you should be fine.
2] Possible Voltage Shortage?
The voltages and battery life responses for all batteries are going to have slight difference. For the most part this won't matter. Most projects that use batteries are not terribly voltage dependent. They will either boost or regulate their voltage to get the voltage they want out, or they will be able to run at a wide range.
As a note, "Shortage" in this context usually means you are creating a short across your battery. Might want to be careful with that terminology.
3] Fundamental Reason for this Voltage Range
I am not an expert on this, but I know it deals with the chemistry of the battery itself.
4] Parallel Cell Charging - One BIG Li-Ion Battery Pack
This can be done. There are some issues that can come up when doing it. This might be worthy of a question by itself. If you do ask, might want to ask if the same can be done for packs in series.
5] Charging... How?
Same as previous answer.
2 1.8Ahr cells in parallel is the same as 1 3.6Ahr. Its called a 1s2p cell.
Sometime LiIon chargers stop charging after a fixed time. Something like the capacitry / charge rate * 1.5. So you might not end up with a fully charged pack after a single recharge.
Using a single cell LiIon batteries in parallel (with the same specifications and age) is generally acceptable.
When they are initially connected though, you need to make sure they're nearly at the same voltage. When connected the current flow will be I = (V1 - V2)/(ESR1 + ESR2). So long as I is less than the peak discharge and charge current you're fine.
If the batteries have the same specs, there is nothing more to worry about.
If the batteries are different (but operate over the same voltage range) then one just needs to make sure that during charging the current sharing doesn't violate the charge rate of 1 cell.
Even have a multicell stack, putting them in parallel is fine. But here it is a bit more tricky as the stack needs to be connected at the cell level, and the parallel capacity of each cell in the stack needs to be nearly identical.
Best Answer
As long as you connect them correctly, this should be no problem.
You need to connect the point between cell 1 and cell 2 of battery pack A to the point between cell 1 and 2 of battery pack B. Connect the point between cell 2 and 3 of A to the point between cell 2 and 3 of B.
And so on.
What you'll end up with is a 5s2p battery that can be balance charged in the same way as a 5s1p battery.
And of course, connect the very ends of pack A to the corresponding ends of pack B as well.
Before you make a connection, you should ensure that both connection points is at the same voltage potential (in other words, before you hook up the point between cell 1 and cell 2 to each other, check that cell 1 in battery pack A is at the same voltage as cell 1 in battery pack B).
You don't want huge amounts of current to flow to even out the charges as soon as you hook the cells up to each other, because that can become hot and/or damage the cells.
At this point, the charger should not know or care whether it's a 5s1p battery or a 5s2p battery, other than that the capacity is roughly doubled.