If the dispute is between housemates or roommates, you need to stay out of the matter as much as possible. Let the tenants know that they chose to live together and each of them is individually responsible for the entirety of the lease agreement. Hopefully they will continue to get along with each other until the lease expires or until one of them can find a replacement for the other.
If the dispute is between people in neighboring units, you need to quietly listen to the complaints from each party to determine who you think is responsible for the problem. Next. you need to determine if illegal activity is taking place or if the problem is simply a personality conflict.
Suggest that the tenants see a mediator when the problem is purely based on personality and temperament. You, as a property owner, should stay far away from such disputes. Reasonable tenants may be apt to work out the dispute and even become friendly. A mediator can also point out when the problem is irreparable.
Once you know what the cause of the tenant-to-tenant dispute is, you need to determine what you can do to alleviate the problem. If there are illegal activities going on, you need to encourage neighbors to report what they see to the authorities. If there are activities taking place that violate the lease, you need to ask that the tenants to stop. In the worst-case scenario, you may have to serve an eviction notice to tenants who are causing harm to the property or causing you to lose good tenants.
Once you have dealt with the majority of the ongoing tenant-to-tenant problems, you may want to address certain behaviors in your lease agreements, especially if the issues are recurring.