Where did it become known that Rin's and Len's names have different first letters?
The earliest archived revision of Crypton's page for the Kagamines that reveals details about them has their names in both Latin script and in Japanese. 



That revision is from 3 Dec 2007, which is shortly before they were actually first sold later that same month. 

So I guess the answer to your question is that it was known to people pretty much from the very beginning.