One can run more than one My Cloud device on their local network. Just give each device a unique name. The My Cloud online portal should work with multiple devices. There should be an option to switch between each of the devices in one’s My Cloud online portal, provided one used the same email account for cloud access on each My Cloud device.
As Bennor said give each one its own name. I have two WDMYCLOUDS. My first one I named WDMYCLOUD and when I bought a second one, I named it WDMYCLOUDTwo.