I'm not certain what shred again does. Does it copy the ability from the stack? After it resolved? What about targets? Can they be re-chosen? There is not much of a point to doubling down on the same creature.

Does it "copy" them by gaining them? Attacking makes tap abilities unlikely...

I also think the concept is getting additionally complex due to both shred again and the activated ability itself targeting a creature here.

Yeah, choosing new targets will be necessary and the original wording did not include that.

The ability activates upon the targeted creature attacking. If the wording does not reflect the intended way the card should work, please let me know, and if you have any suggestions that make it clear attacking causes the ability copy, please let me know.
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As for this card's individual targets, the idea is just to give two creatures an evasive ability. I would hazard most board states, the player can remember which individual cards are targeted, but if they had a really large token board, I guess just some kind of object as reference, which is probably already done. ­
Whenever ~ attacks, target creature you control can't be blocked by creatures with a power of two or less until end of turn.
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This should turn the activated ability into a triggered ability on another creature where attacking is the trigger.