There are very expensive $250- $300 chargers that do higher than 6s, probably closer to 10s, but not a lot to choose from and right now my memory is drawing a blank on their names, but they do exist.
I have no idea why there are not more, but I suspect that the demand is simply not there yet. Lithum batteries at those higher voltages are not as common and can be very expensive as are they chargers.
The charger you link to balances its battery by having a charging connection and balancing connection at the same time. The charging connection is directly connected to the + and - of the battery and supplies the main charge. The other connections are more complicated and, for example, in a 6s battery there would be 7 connections, one at the "-", on at the "+", and a connection or wire coming from every single cell connection. So each time another cell is added to make it a 2s or 3s, a wire comes out between the "+" and "-" of each cell added. So a connection between all 6 cells and one at the botom or "-" and one at the top or "+" and you have 7 wires coming out that will then plug into the side of the charger.
The charger then monitors each individual cell's voltage as it is charging the battery as a whole, but most chargers don't seem to start balancing until the battery is essentially full, or at least one cell is at 4.2 volts. Then it uses the seven wire connection to balance the battery, usually by discharging the higher voltage cells a little via a small current, and then charging the whole battery again slowly. Then repeat until all balanced.
It looks like what you linked to would work, except that they are for smaller number of cells in series than what you want to do.
Another option that would work for you is to do what you are doing - split the 10s into 2 5s and charge them independently, but parallel charge them using a parallel charging board and then you could charge them at the same time.
Check this out: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14856
Edit: see also Is soldering wires directly on a NiMh battery safe?
nothing in there that I could see that would 'balance' them
This may be why they don't last very long.
Due to manufacturing variance, one of the batteries will go flat first while the others have more charge. This can result in current being driven through it by the others while it's flat, damaging the battery. Or the reverse happens while charging: one will fill up first, and then suffer overcharge damage. Does the charger automatically stop on full charge or it it timed?
It may be better to adapt the battery holder to the batteries with a saw, so you can use the holder and charge them individually.
Or look for cheap repurposeable packs, eg. http://www.dx.com/p/gd-509-2-4v-1200mah-rechargeable-2-x-aa-cordless-phone-replacement-battery-pack-white-292547#.VIXLYsmfjjU is clearly 2x AA heatshrunk together, you may be able to find 4 heatshrunk together in the right format then just change the connector.
Edit 2: by the way, I would put money on the tanks suffering mechanical failure or enemy action before the battery packs become an issue. Unless they're expensive 'models' rather than 'toys'.
Best Answer
Lithium Ion cells are MORE susceptible to "vent with flame" / "magic smoke" events than LiPo (Lithium Polymer), not less.
If you want a much safer Lithium based chemistry cell then use LiFePO4.
(LiFePO4 = Lithium Ferro Phosphate).
You can drive a stake though their hearts (metal or not, at cross roads and at midnight optional) and they will not produce high energy discharges. They will (probably) not work very well thereafter
LiPo and LiIon have similar charging needs.
LiPo usually allows larger charge rates but is essentially the same chemistry.
A charger must "respect" the maximum battery voltage and maximum charge current - these are usually selectable. The per cell termination voltage for LiIon or LiPo is usually 4.2V at 25 degrees C. Using a slightly lower voltage than this reduces capacity but greatly enhances cell cycle life.
A LiPo or Lithium Ion cell provides from 3.0 to 4.2 V/cell (discharged to fully charged) and cells are rated at 3.6V or 3.7V nominal A LiFePO4 cells provides about 2.8 to 3.6V cell and are rated at 3.2 or 3.3 V nominal. (Lower is possible but there is little capacity in the "tail" and using it reduces cycle life). A NimH cell produces about 1.0 to 1.3 V/cell and is rated at 1.2 V nominal
So your 10.6V NimH pack = 10.6/1.2 = 9 cells
Across charge range:
You can stop discharging the LiFePo4 cells at somewhat higher than 2.8V so that the range is say 9V - 10.8V
You will need to determine how low a voltage the servos will run on happily but the LiFePO4 cells will probably be acceptable and are far better for safety. That said, properly run LiPo cause reasonably few problems.
Using 4 x LiFePO4 cells could be considered.
You may wish to consider the use of a 12V lead acid battery. These are cheap, a wide range of capacities are available and chargers are readily available.
You could use a boost converter from a lower voltage or a buck converter from a higher voltage to get good constant voltage operation.
Balanced charging
LiPo cells can be charged using readily available "balancing chargers" or charged separately in parallel or you can make your won charger. A range of balancing charger ICs are available from eg Digikey.
A simple method to ensure balancing is to place a clamp regulator across each cell which shunts current when voltage reaches the constant voltage charge point. There are various ways of doing this but one such is shown below. This is from here. Good notes on operation and adjustment are shown on that page.
The IC monitors battery voltage and when a trigger voltage is reached the MOSFET is switched on to shunt current. The IC contains an internal voltage reference and provides programmable hysteresis for the clamp voltage. Use of one of these circuits across each cell allows use of a non balancing charger up to the current capacity of the clamp.
Black belt holders only In fact, although this would not be recommended, you could probably use a simple 12V lead acid charger with a current output no higher than the capability of the clamp circuits to charge 3 x LiIon or LiPo cells ion series. This would not provide some of the finer points of LiIon / LiPo charging but should work.
MAX921 datasheet. These are far too expensive at $4.60/1 Digikey.
Same result can be had with a TL431 and maybe a few transistors. A workable circuit can be arrived at if these is enough interest.