I'd like to know what happens if the RTS frame will be transfered by two devices (which cannot hear one another) in the same time. So it's like a classical hidden terminal problem - A and C are not able to see each other and want to transmit to B. I know that RTS/CTS reduces the collision problem so is it one possibility when the collision will appear?
A collision happens when two devices transmit at the same time, so, yes, a collision occurs, even if the two sending devices cannot detect the collision. It is, after all, CSMA/CA (for Collision Avoidance). The WAP will dtect the collision, and it will not send CTS to either requesting host. Both hosts will try again, later.