Did we ever make a final decision about whether a Spirit can possess an allied Spirit? I think that @HorribleTomato, the inventor, is the right person to make the decision.
Spirit Explained

I wouldn't think so. The lone Spirit can't capture. So a knight possessed by a spirit does not count as two attacking pieces. I wouldn't find that logical.

Can a witch posessed by a spirit capture like a knight.
Yes because the rues say: "A possessed piece can move and capture like a knight in addition to its normal movement"

Fairy piece interactions:
If a spirit possesses a witch, the witch just moves as normal and can not capture.

Fairy piece interactions:
If a spirit possesses a witch, the witch just moves as normal and can not capture.
Is this something more general? Is it that any piece, that has no own capture ability, does also not get capture ability from the Spirit?

Can a possessed pawn move as Knight to the 1st rank, and can the Spirit leave the pawn alone there?
If yes, will the pawn have a 2-move from the 1st rank?

This is a very innovative piece. Lots of good thoughts went into it.
My only suggestion is not use the Witch icon, for obvious reasons: it's all too easy to confuse, even if it's a flipped mirror image. (I've seen horse head icons face both left and right; it's still the Knight piece, either way.)
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As an extension of Evert823's question: are possessed pawns promotable?
I have a pretty good idea what the answer might be; I just needed to run it by you, for clarification.

As an extension of Evert823's question: are possessed pawns promotable?
The OP here has an example of a possessed pawn that promotes to a possessed Queen. I don't think that there is any doubt about this.
However, my question is if a possessed pawn can move to the own 1st rank.
So in this diagram, can White play Ppe1 and can black play Ppf8? And what happens if Ppe1 and Ppf8 are later followed by Sf3 and Se6 respectively?

Ah, the example wasn't in this thread. (For a second after reading your response, I thought I lunkheadedly missed something that was clearly stated. haha)
I can only guess that the answer to your original question is no. Regardless of where a pawn is on the board, if it has already moved, then for the rest of the game, it has waivered its ability to move forward two squares. A possessed pawn already moved, if it's reached the backest rank by Knight maneuver.
But, of course, this is just my educated guess. Martin will hopefully give the official answer.

Update:
Changed:
"A possessed piece can move and capture like a knight in addition to its normal movement"
to
"A possessed piece gains the ability to move and capture like a knight"
Changed:
"If a spirit possesses a witch, the witch just moves as normal and can not capture."
to
"Pieces without any normal capturing moves will not gain capturing knight moves from the spirit. For example, if a spirit possesses a witch, the witch just moves as normal and can not capture."
So, if a spirit possesses the king, the king can move like a knight. Doesn't that make him extremely hard to catch?
Yes, the king will indeed become hard to catch. But I'm not sure if it is really a big problem, since it most cases I think the spirit is better to use offensively, rather than defensively. If we get to endgames, then the spirit probably rather possess pawns to promote, rather than possessing the king.
This has never happened before, but in theory a queen and king vs spirit and king should be unwinnable. So I guess that goes to show how powerful spirit is on offense and defense. Maybe too powerful? I don’t think enough games have been played to gauge if a spirits strength in keeping the king alive is too annoying and drags games too long or to too many draws.