Starcraft – ny benefit to pre-purchasing Heart of the Swarm

If I'm understanding your question properly, in order to have two Heart of the Swarm licenses you need two Wings of Liberty licenses.

Since Heart of the Swarm is not a stand alone expansion, you need to have all the prerequisite games in order to play it (each copy).

You can see for example, on this Cataclysm box art for World of Warcraft, that it requires "Wrath of the Lich King to play". Wotlk says it requires TBC, and TBC requires the base game. It is expansions all the way down. This trend stands true for all Blizzard games to date, as they don't release stand alone expansions.

I haven't heard of any announcement declining or supporting such a limited availability, considering it's digital only (so nothing to produce and store somewhere).

Especially considering there are skins unlocked by the DDE/CE that alre also listed for People only owning the Standard Edition (like me), I'd assume they wouldn't limit that supply, even if it's just for those "completionists" who'll buy the upgrade just to get everything.

Right now they offer the DDE upgrade for 15 € (possibly similar price tag for other regions; only available if you already own the game).

My personal opinion: That price just for some cosmetics isn't really something you should worry about. For 15 € you can get better stuff than that. If you're not interested in the physical CE, just grab the Standard Edition and play the game. Later on, if you still think you might want the DDE, grab the upgrade. Worst case (they stop offering it), you'd still have the full game, just minus that skin stuff.