I did my 2 battery mod a while ago...but having to cut out my old chassis back window to make the battery stick out was not cool nor did it look cool...and besides that having a battery on the top of your car seriously raises the center of gravity makeing the car more prone to fliping.
Well after long hours

of grinding with my dremel I got the 2nd battery into the chasis (a lot of modifying!!!) and now it is totaly sweet. The car is as fast as before I can use normal chassis(no battery stickin out) and the center of gravity is even better that before the mod was done...more weight on bottom of the car lowering the center of gravity...now the car is liek a dream and looks normal...
I would love to post pics and if you have any questions ask...but I have no camera...