Having a device too close to a gateway is known to overload the front end. This can cause phantom fake signals to appear to be received on channels other than the true one.
Network servers have gotten smarter about ignoring the fakes and responding to the real signal, but its possible that having phantoms appearing on two gateways might still somehow frustrate the algorithm.
Being too close to the gateway can also cause even the “real” signal to be corrupted, so you want to avoid that.
Keep your nodes and gateways further from each other. If you must have a “bench test” gateway, rig it with a termination resistor rather than an antenna.