Just wanna share my experience of dual battery setup...
Recently my HL Panther Tank got some problems with it's original battery pack so in country like Singapore it's hard to find a replacement set rom my local shop, therefore I decided to DIY build a battery pack.
Original was 1700 MAh and now it's a whopping 3200 MAh capacity battery pack was born. As for my new battery pack I actually connected it in "parallel" (1600 MAh x 2 = 3200 MAh). BUT do take note that if you connect the batteries in series connection instead of parallel, then your precious control board in your RC tank may fried / burnt...

Now my tank panther can last more than 1 hour...
Nevertheless since with dual battery setup can extend your running with the tank, but do take note that you will also have to charge it longer unless u have a Smart Quick Charger that is compatible with your new battery setup.

Unless you are charging the each of the dual batteries with a seperated charger, otherwise
as thumb of rule, try to AVOID mixing 2 different capacity in the dual battery setup (etc. 1600 MAh + 2100 MAh), because during charging the lower capacity battery could be over-charged or the higher capacity battery could be under-charged.
So technically ===> "Longer Running Time = Longer Charging Time"