Not sure how to handle tenants clashing

We have tenants constantly complaining about a neighbor’s dog barking late at night. The twist is, the neighbor is also a tenant. It quickly turns into a he said/she said scenario, and I don’t want to take sides. Plus, it's a family dog & old, so I do not want to be insensitive either.
What do you do in these situations? Do you set strict rules, involve written warnings, or try some kind of compromise?