There is a file called rtl_eeprom.exe that you can use to change the serial number, which can be downloaded here:
Index of /binaries/windows/rtl-sdr/
Scroll to the bottom of the list and download the most recent 64-bit version onto your C:\ drive in Windows. Create a new folder on your C:\ drive and unzip all the files into the folder, including the .dll’s. Plug in one dongle and open a command prompt in Windows and navigate to the folder where you unzipped the files. Then run the following command to change the serial number from the default of 00000001 to 00000002:
rtl_eeprom.exe -s 00000002
Review the configuration information and enter y to make the change:
Now you must unplug the dongle and re-plug it back in for the change to take effect. Plug in the other dongle so that they’re now both plugged in and run rtl_test.exe at the command prompt to ensure the dongles are now 00000001 and 00000002:
Terminate the .exe from running by pressing Ctrl + c.