Does Defiant Dance need to target 2 units? Can they target the same unit?

Yes, Defiant Dance must target two different units.

According to the card's text, it instructs you to give a unit +2 [S] and "another unit" -2 [S]. As confirmed by RiftJudge FAQ #2956, the word "another" indicates that the two targets must be distinct; therefore, you cannot target the same unit twice, nor can you play the spell if there is not a second valid unit available to target.

Additionally, per RiftJudge FAQ #2002, you must be able to select two distinct valid targets to play the spell.