Get rid of String (capital S) and use one of the approaches in serial input basics - updated to directly get data into a character array.
https://www.arduino.cc/en/Reference/StringToCharArrayYou could, of course, work with char arrays in your program to avoid the need to convert from String to string
char_array should give me 0xB20000 char_array should give me 0x000DFF
char_array is the first element of char_arraychar_array is the second element of char_arraySerial communication has no idea about how the data is created (String, binary stuff, character array). So yes, the Serial Input Basics - updated thread will work; even contains an example how to parse (split) the data.