Can you spend Sett Brawlers Buff then give him another Buff from Pit Rookie then spend that second Buff?

Yes, you can perform that sequence. Because Sett, Brawler's ability to spend a buff is an activated ability and Pit Rookie's ability is a triggered ability, you can chain them together as long as you have the necessary resources and timing.

Here is the sequence to achieve this:

1. Initial State: Sett, Brawler is on the board with a buff.
2. Spend First Buff: During your Action Phase (when the chain is empty), you activate Sett, Brawler's ability: "Spend my buff: Give me +4 might this turn."
   • You spend the buff (removing the counter).
   • Sett gains +4 Might for the turn.
3. Play Pit Rookie: You play Pit Rookie from your hand.
   • Pit Rookie enters the board.
   • Pit Rookie's "When you play me" ability triggers and goes on the chain.
4. Resolve Pit Rookie: The chain resolves. You target Sett, Brawler with Pit Rookie's ability.
   • Since Sett no longer has a buff (you spent it in step 2), he receives a new buff counter.
5. Spend Second Buff: Now that Sett has a new buff, you can activate his ability again during your Action Phase (when the chain is empty).
   • You spend the new buff.
   • Sett gains another +4 Might for the turn.

Important Rules to Note:

• Rule 702.3: "There can only be one Buff on a Unit at a time." This is why you must spend the first buff before Pit Rookie can successfully apply a new one.
• FAQ #1477: Confirms that Sett, Brawler's ability can be activated multiple times in one turn to stack buffs, provided you can give him a new buff between each activation.
• FAQ #5784: Confirms that Pit Rookie's ability targets another unit, and the "it" in the reminder text refers to that targeted unit.