How to ship a container and use container check box on Equipment screen

How to use Container check box on Equipment screen to state a container as an equipment and at the same time I have to define another Flat Truck as a equipment too, Then the main question here is how to assign that container on the flat truck knowing that we are now having 2 equipment one for the container and the other for the flat truck and the power unit can be assigned by only 1 equipment at the same effective time.