Out-of-universe, which name for the lead villain did Rowling come up with first? Did she always intend to have two names for him? Looking for official sources only.
We don't know
We have access to very little of J.K. Rowling's original manuscripts, and what we do have sadly doesn't really tell us much.
Of course, in the published books "Voldemort" appears in book one, and "Riddle" doesn't appear until book two. Barely any of our early manuscripts even contain passages with any of those names.
Only three of the released manuscripts seem to mention Voldemort at all.
"Red-eyed" - early 1990s
Lord Voldemort - early 1990s
"Voldemort" - 1995
It's possible that either name came first, but without access to more of Rowling's manuscripts we really can't say. We don't have the amount of available infomation that we have with Tolkien's writings.