You just need to replace the one NOX sensor unless they are so far out of spec that the second one throws a code shortly after changing the first one. This happens frequently, so in my case I just changed them both.
4595 is smooth running controller cylinder 6. It might be indicative of a faulty fuel injector. Does your bulkhead partitions leak? See this thread
Error code 4B99 and 483D - Bimmerfest - BMW Forums