The WD settings are in the configuration folder, file name device_drivers_SMART_DEVICE_APP.py.json. or device_drivers_SMART_DEVICE_APP.py on an older system.
My guess is that you are using different ports and one is being blocked. Another guess: somewhere along the way you set IP addresses for the one that isn't working. You can check that by going to calibre Preferences / Plugins, selecting the Smart Device then pressing Configure.
An even better idea: run both in debug mode and compare the SMART DEVICE debug lines. You will see the IP addresses and ports it is using. If they are different then change the failing one to use the other info. If they aren't different then the mystery deepens.
