You can run the adapters in series (so long as they are the same type with the same number of AA cells and that the cells match each other in state of charge and capacity). The problem with reverse charging comes in when you drain rechargeable batteries down and some still have a charge and some don't. Best not to try to drain the cells too much and charge them before they get too dead. Reverse charging NiMH's won't blow up, but it dose tend to damage the cell(s).
You can run into the same problem with NiMH D cells, the only difference that there are usually 2 or 3X as many AA's used in adapters, so you increase the chances that one of the AA cells may get reverse charged.
Reverse charging Li-ion cells can be very dangerous, particularly when you go to recharge them after they have been over discharged - this is why it is best to use protected Li-ion cells in series (the protection will shut the cell down when the Voltage tarts to get too low). Protected LI-ions are also safest in single cell devices that lack built in low Voltage cutoff.
Goodness, I'm glad I'm learning this now! So just to make sure I understand (and to learn a little more), if I had a flashlight with 3 D cell nimh's and say the middle one became too low, you're saying it would start charging in reverse? From the other cells?
Or does this only happen when you have them hooked up in like parallel groups.
My knowledge of electronic circuits is still in its infancy, but the first thing that comes to my mind would be to add some kind of diode between the cells to prevent backwards current flow.
Or does there exist a "protected" nimh?
Definitely good with lithium, I only buy protective cells because I just feel better with them. Only downside is that the high voltage is sometimes a problem, I know generally the higher the voltage the better in terms of efficiency but it's not always realistic to find bulbs that work on very high voltages. Haha
