This is likely related to how an Apple TV is identified on your network. When only one device is in use it can be accessed using ‘AppleTV.local’. If you add a second device, it will likely have a name like ‘AppleTV-2.local’.
There are 2 ways to resolve this.
Use the Apple TV IP address (visible on the Apple TV in Settings --> General --> Network) in place of AppleTV.local.
Change one of the Apple TV's hostname in nitoTV > Settings > General. Change this from AppleTV to AppleTV-Office or whatever you want. When connecting, just add '.local' to the end of the name you chose to connect. Note: A restart of the Apple TV will be required for the settings to take effect.