Containers are used for picking objects together, you still need to give both objects the action.
This is useful for when you have conditions that trigger on one or the other, and you want your set of actions to affect only those specific instances that are grouped together in a container.
For example, if you had an "on collision" type condition with the tank base object, and you had the action make tank turet flash, under normal construct logic all tank turrets on the screen would flash, because no specific instances were picked by the condition. If you had the two in the same container, then the tank turret associated with that tank base would be picked, and the action would apply only to that turret.
You should still utilize the pin behavior for the effect you are looking for.