It's possible that this is just a coincidence. When two or more events coincide by chance without any connection to one another, this is called a coincidence.
The fact that their houses were separated by a small crack in the wall may have been coincidental in this instance. Instead of being the result of any deliberate action or intention on the part of the homeowners, it is possible that the crack developed as a result of natural factors like the foundation's settling or changes in temperature and humidity.
However, the situation might be regarded as ironic if the homeowners had purposefully caused the wall crack as a means of communicating with one another. Irony is when something goes against what is expected, or when what is said or done is different from what was meant or expected.
For instance, it would be ironic if the homeowners had purposefully created the crack as a means of communication because a wall is typically thought of as a barrier or divider between people; however, in this instance, it was being used as a means of connection or communication. If the homeowners had done this, it would be ironic.
Hope this helps, have a great day :)