I don’t know if it is written somewhere in the documentation / user manual or not. But it bears repeating in any case.
I just installed my 2nd WDTV Live in my home, and had no end to problems… until I renamed the device. Go into the setup and rename your 2nd device to something like “WDTV Live2”, and that will do it. You must do a “Device restart”, but it works a lot better after that.
I think the device name is under Network Setup.