Can anyone tell me: when renting a car, the trunk must fit two suitcases. But if there are four suitcases, can the other two suitcases be placed on the back seat or the gap between the front and back seats ?

Ordinary cars can hold four boxes.

Of course, just be careful about theft.

Rented a car, the car is yours to use as you please, you can install anything you want in the car, regardless.