Some RC cars or model kits allow to add the steering of a second (mostly rear) axis, using a second servo. This is, for example, true for the Tamiya CR-01 or G6-01 chassis. Tamiya recommends using a remote control system and an additional channel to set this up. If you don’t already own such a remote, it can get pretty pricey to get a new one which has this feature. Or maybe, you’re remote is capable to do this, but you want to save the channel for a winch or some switchable lights.
The advantage of using a second RX channel for steering, is, that you are able to trim and limit the steering angle by using your remote’s settings without the need of touching the hardware anymore.
The other option is to use a so called servo reverse circuit or servo reverser. Usually the come as a small circuit board and some headers, to connect the servo(s) and receiver to. They are also sometimes used on RC planes.
There is a third option, that I don’t want to forget to mention: Some servos allow to be rewired (basically by swapping the motor wires). But this only works if the servo uses wires internally — if the motor is soldered to a PCB this doesn’t work.
Do I need a reversed signal?
The first thing you should check, is, if you actually need to change the direction of the second servo. For example, the Tamiya G6-01 chassis is made that way, that you don’t need a reverse signal. The servos are already mounted that way, that they turn into the right direction. So basically you can just connect the seconds servo to the first one (the signal line, that is).
Then you need to decided which of the two axis you want to trim using your remote. If the other one isn’t centered while the first one is, you need to adjust your hardware until it is. This is the cheapest way and for the G6-01 it seems also the best way, since the rear steering is already geared down by about 50%. You only lose the ability to easily adjust both axis with your remote — but I guess that’s acceptable.
The Tamiya CR-01 on the other hand needs an reversed PWM signal for the seconds (rear) servo, since it is not possible to mount it any other way. Please note, that simply turning the servo around its turning axis will not reverse its direction. You frame or chassis may allow for a completely different mounting position, but at least the CR-01 chassis does not.
Roundup: If you don’t need a reversed signal, you can simply put both servos in parallel (same channel) on the receiver. If you want to change the signals independently, like the range or the endpoints (trim), you still need additional hardware, though.
Using a servo reverser
A reverser is a simple piece of electronics that reads the actual PWM signal from the receiver and outputs the reversed signal. Simple reversers will do just that and nothing more. In this case, trimming needs to be done, by adjusting the steering rods. Unfortunately, there is no way of limiting the movement range, if you use these kinds of reversers.
Some, more advanced reverser circuits, allow for Dual Rate setting and/or even trimming using some onboard potentiometers. Dual Rate allows you to reduce the maximum value for both directions at the same time. This allows for a reduced steering angle on the rear axis.
There is even the possibility of programming and using a Atmel ATTiny13A micro-controller to build your own reverser. But note, that I’ve not tested this, so you’re on your own with this solution.