I want to build a custom solution for controlling a 4 standard DC motors with an Arduino UNO and I don't how to proceed. It'is possible to use 2 motors shield ? Thanks
If you mean shields in the strict sense of the word where they plug into the Uno, then probably "no". Assuming you could physically plug the second one into the top of the first one, they would both be expecting to use the same pins.
But if you mean breakout boards, then probably yes. Look at the Pololu 2130 for example: needs 2 data lines per motor so 4 per breakout and thus 8 data lines for 4 motors. It's very small and I think you can fit 2 on a mini-breadboard.
Not saying it's necessarily the one you want: check its specs for motor voltage and current. Pololu have quite a range of motor power components.