Does Defiant Dance fizzle if you remove one of its targets?

No, Defiant Dance does not "fizzle" (fail to resolve entirely) if you remove one of its targets.

According to Rule 359.3.e.5, a spell resolves even if some or all of its targets become illegal. Furthermore, Rule 359.3.e.8 states that if an instruction has multiple targets and some become invalid, the instruction will execute on the remaining valid targets.

Breakdown:

• Targeting: Defiant Dance requires two distinct units to be played. You must choose both targets when you play the spell (RiftJudge FAQ #2956, FAQ #8790).
• Resolution: If one of the two targets is removed from the battlefield (e.g., recalled, destroyed, or moved to a non-board zone) before the spell resolves, it becomes an illegal target.
• The Outcome: The portion of the effect targeting the missing unit fails to execute, but the portion targeting the remaining valid unit will still resolve normally (RiftJudge FAQ #9004, Rule 359.3.e.5).

Note: This is based on standard targeting rules and specific FAQs for Defiant Dance. If a target becomes illegal, you cannot re-target the effect to a new unit; the effect simply fails for that specific target (RiftJudge FAQ #8790).