I am the landlord. I have two tenants that claim each other are messing around with the hot and cold water while they are taking a shower. Any ideas on how to resolve this. Or do I need to legally?

Define messing around. Is someone actually turning off the water or is it a normal pressure issue from two people using water at the same time?

Both tenants claim that the other is intentionally turning the hot and cold water off and on to effect the showers of the other. In 50 apartments and 17 years this is the only incident I have encountered.

Personally. I'd tell both to grow up (maybe not THOSE exact words however, be more descrete). There is nothing that can be done about it.

I will add if I was a tenant and I had such poor water pressure that someone in another apartment could effect my water, I would be complaining myself