Zuxxez's VP of Business Development Dirk Hassinger revealed in the official forums that Two Worlds: The Temptation does not re-used a single 3D object from the original, and that due to its many enhancements (new multiplayer netcode, new gfx & sound engine) The Temptation is not even compatible with the original. Therefore, it has to be considered a sequel, not an add-on. Two Worlds 2 just didn't sound right.
Here's a translation of Hassinger's original German post in full:
"I find the discussion going into the wrong direction. As I've already said Two Worlds: The Temptation becomes a completely new game and not an add-on. Not one 3D object from Two Worlds is included. That couldn't be done due to technical reasons. We use new graphics, sound and engine. Multiplayer also has completely new codes. And it cannot be an add-on, because it is not compatible with the first Two Worlds. We did not call it "Two Worlds 2" only because that sounds strange to us.
Nevertheless simply compares the screenshots of Two World: The Tempation with Two World. (I know that there are only 4 on the Internet.) There will be new sceenshots in the next PC Action and the next GameStar. In addition we're presently working on a web page. Until those are ready, [we] give weekly information in Antaloor Post. You can read those yourself on www.2-worlds.com."