Use containers: this will ensure that each mob gets a collision box when it's created, and when one dies, so will the other.
The simple method to pin a container is with the pin behaviour. So do something like this: when the mob is created, position the collision box to it, and then pin the box to it (two separate actions, same condition). With containers, the box is automatically created when the mob is created, and the correct instance of the box is automatically picked when you pick the mob.
Pinning and assigning position every tick are identical in terms of functionality and resource usage.