Its player 1 turn and they played a spell targeting player 2 unit can player 2 play 2 reaction cards at once?

Yes, Player 2 can play two reaction cards back-to-back before passing priority to Player 1.

According to Riftbound FAQ #4696:

1. Retention of Priority: After Player 1 plays an action (the spell), Player 2 receives priority to respond. When you play a reaction card in response, you do not automatically lose priority; you retain it.
2. Chaining Reactions: While holding priority, you can immediately play a second (or third, etc.) reaction card on top of the chain before passing priority to your opponent.
3. Passing Priority: Priority only passes to Player 1 once you explicitly choose to pass it.
4. Resolution: Once both players pass consecutively, the chain resolves in LIFO (Last-In, First-Out) order. Your second reaction will resolve first, followed by your first reaction, and finally Player 1's original spell.

Sequence:

• Initial Chain: [Player 1's Spell]
• Player 2 acts: Plays Reaction A.
  • Chain: [Player 1's Spell] → [Reaction A]
• Player 2 retains priority: Plays Reaction B.
  • Chain: [Player 1's Spell] → [Reaction A] → [Reaction B]
• Player 2 passes: Priority passes to Player 1. If Player 1 also passes, the chain resolves starting with Reaction B.