Hello, this is an issue that drove me mad for a long time. My setup is rather simple: KPA unpowered, 2 Yamaha DXR10 connected in stereo with 2 TS cables (monitor out and direct output), that's it.
At home, a slight noise appeared when connecting the second Yamaha to the direct output, fixed by pushing the relative ground lift button. In live venue, this wasn't that simple: sometimes, when someone turned on some other equipment (speakers, lights and so on) a very loud buzz came from the DXR10, making me struggling to find the right combination of lift buttons.
To solve this issue, follow the DXR10 instructions to link a second speaker: connect two TS cables to the second input of the first Yamaha (Left and right). Push the stereo button on the bottom and connect a XLR cable from the link out to first input of the second DXR10; set the volume to 0 and the link mode to mono. Mic/line switch set on line for both speakers.
The second DXR10 volume depends on the first DXR10 volume, so to balance them or to make one speaker louder, adjust the two volumes from the KPA.