Elsewhere, the tenet "Orion needs to be rescued" has been proposed as a good seed tenet, a starter to get people thinking about what the game will be about. But I was wondering: is that one coin or two? It seems to me that it would have to be two coins: one to create a thing called "Orion," and the other to add to it the trait "needs to be rescued."

But if it's only one, then why?

Yes.

Or perhaps more helpfully, that's the standard each group establishes for itself.  There's a fairly elaborate example about shooting a gun or fingers pulling triggers in the book on this because it is a pretty key issue.

In play, a player says "Orion needs Rescued" and pushes forth some number of Coins.  If nobody says boo about it, then you've established one data point for future reference as to how your group will be charging for things.  If they push forward one Coin and you say "Hey, shouldn't that be two..." and they say "ok" and push forward a second, then you've established a data point for future reference as to how your group will be charging for things.

Note:  That's assuming that Orion has not already been created as a Component.  If I were in the game, yes, I'd push for 1 Coin to Create Orion, one to add the "Needs Rescued" Trait to him.  If he has, then adding the Trait "needs rescued" to him would typically just be 1 Coin.

Since you're talking Tenet phase, I'll point out there is a gimmick in the book on relaxing the 1 Coin per player only rule during the Tenet Phase.