Can I react to Mind Splitter with Star-Crossed?

Yes, you can react to Mindsplitter with Star-Crossed, but there is an important distinction regarding what you are reacting to.

Ruling

You cannot react to the act of playing the Mindsplitter unit itself, as it enters the battlefield immediately upon being played and does not use the chain (Rule 356.2). However, once Mindsplitter is on the board, its "When you play me" (WYPM) triggered ability is placed on the chain. Because this triggered ability uses the chain, it creates a Closed State, allowing you to play Star-Crossed as a reaction to that ability.

Sequence of Events

1. Opponent plays Mindsplitter: The unit enters the battlefield immediately.
2. Triggered Ability: The "When you play me" ability is placed on the chain as a pending item, creating a Closed State.
3. Your Reaction: You play Star-Crossed as a [Reaction], adding it to the top of the chain. You must target both a friendly unit and an enemy unit (which can include the Mindsplitter itself).
4. Resolution (LIFO - Last-In, First-Out):
   • Star-Crossed resolves first: It returns the targeted friendly unit and the targeted enemy unit to their owners' hands.
   • Mindsplitter's ability resolves second: The ability attempts to resolve against its remaining targets. If the enemy unit targeted by the ability was returned to the hand by your Star-Crossed, that portion of the ability fails to execute.

This is consistent with RiftJudge FAQ #10299 and #9017, which clarify that while units cannot be countered upon entry, their triggered abilities placed on the chain are valid targets for reactions.