Each servo is on a different channel, meaning they can each be controlled independently.
On your remote control, one button can be used for forward motion, while another can be used for steering. They can each operate at the same time, yet do different things.
It is also possible, if you wire it in the right way, to get two servos to operate together and do the same thing at the same time (just connect the signal wires together).
Hope that helps . . .