This is not how temp hp work (in the playtest, or 4e, or as best I know 3E, or in AD&D with the Aid spell).
Temp hp which are lost due to taking damage are no more temporary than real hit points. They just never come back.
What you are describing is an effect which, as far as I know, was prominent for barbarians in 3E but otherwise has never been a big part of the game: the character gets a hp boost = to X, then after a certain amount of time suffers damage = to X.
If you're going to do second wind this way, rather than a hit point boost = to X, it should be healing = to X, because that is what makes it a second wind rather than a first wind. Then after a certain amount of time (say, after combat finishes) the character suffers damage = to X.
The obvious issue with this is that it makes second wind much weaker than it currently is. It becomes a "diehard" style ability rather than self-healing. Whereas I assume that what made it popular in the playtest is that it is self-healing.