OK! So I don't need to have 2 shields.
Good to know and thank you very much.
Indeed, I bought the shields on Pololu website and these shields cover up all the Arduino pins (not all but most of them).
So I have one more question: if the pins used are currently the following:
-22 and 23 for the switches
-25 and 26 for the contact sensors
-A2 and A3 for the joystick
-43 to 46 for the ultrasound sensors
-20, 21 and 2 for the MPU
( -A0, A1, 3 (2 normally but I remapped it), 4, 6, 7, 8, 9, 10, 12 for the fisrt motor shield ) -> replaced by the new one.
do you think I will have enought pins to connect 1 shields able to control 3 motors taking into account the fact that sensors are also connected to the Mega?