For the motors to turn forware you need a mixer, or, some radios have the ability to mix as well.
After I crashed my interim UAV platform I moved the electronics to the sumobot chassis and modified slightly into this creepy machine.

I might take a video later.
For the servo mixing, what happens is the following.
Servo 1 is connected to channel 1 (up/down)
Servo 2 is connected to channel 2 (left/right)
Without mixing, when you press the stick upwards only servo 1 will move forward. When pressing left only servo 2 will move left.
With mixing, when you press stick upwards, both servos move forward, when pressing left one servo moves forward while the other backwards, thus creating turning motion.
It is the same mixing as elevons in RC planes.
Now the other way with a microcontroller, would be something like:
Case (stick up)
{
Servo 1 = Forward
Servo 2 = Forward
}
Case (stick down)
{
Servo 1 = Backwards
Servo 2 = Backwards
}
Case (stick left)
{
Servo 1 = Backwards
Servo 2 = Forward
}
Case (stick right)
{
Servo 1 = Forward
Servo 2 = Backwards
}