Well, an alternative might be having your batteries wired into a switch before they go to the ESC (electronic speed control), and then to the motors.
When a battery runs low, you could then flick the switch, and it would start taking from the other battery supply, thus giving you the same effect as the other tank with 2 batteries.
Another option may be adding cells. Depending on what your motors are rated at, you can add cells that will increase run time dramatically. You may want to see about getting some good 3600 mah batteries or the like, if you arent familiar with these., thay hold alot more "juice" than say your average 20 dollar battery. These are a bit more expensive, but you get the choice of building and wiring them yourself to get the desired resluts, or buying them pre assembled to your liking. Sometimes custom building them is a far better option because you can build them in the configuration to best suit you, and the space you have available for the batteries to be put in. If the tank was designed for 1, you may find this very usefull.
The final option is to wire your motors in series. The tank probably is running in parallel right now. When your tank is running in series, your run times can double, because less battery is used since you can now run 2 batteries withought overloading the motors.
For more info on batteries, visit: www.towerhobbies.com
I hope this helped a little. Although I have never personally owned a tamiya tank, I have 5 rc cars right now, and enough to make 4 more in parts. I mostly drive my clod buster, and race my touring car (tc3) at the local hobbyshop.
regards,
MARC