Have you tried welding something really thin between two of them? Like a nail file or butter knife. Sometimes just getting a little air in, breaks the seal and they come apart.

We bought a set of 4 plastic cereal containers. They came stacked with a plastic bag between each container and ate totally jammed together. I don’t think the ice and warm water truck will work because there are four containers stacked. Any other ideas for how to separate them or should I just return them?