So when you say you have a NanoStation between your area, is that just something they use occasionally when they want to connect, or is it their full-time connection? If they’re on the same IP range and using the same gateway and there's no VLAN blocking access to your NVRs then they could just add your NVR’s cameras directly by using their IP addresses. When adding the device, it will show how many channels it has, and if they wanted to, for example, add channel 2, 5, and 8 from one NVR, and channel 4, 8, and 12 from another, they would simply load that NVR’s IP and choose the corresponding channel number for each camera they want to add.
But if they’re on a different IP range and still have full access to the network the other NVRs are on, they could pull the video using an RTSP stream. That even works over the internet if the RTSP stream is open. Since you're using Dahua, it might take some extra steps, but some models can use the Zero Channel over RTSP to display the entire system as one feed. The catch with that is it reflects what’s being displayed on the main view so if the display is in a 16-channel grid, that’s what they’ll see; if it’s in a single or 4-channel view, that’s what will stream too.