If I attack into purple Kha'Zix with one unit and reaction Ambush another unit onto the battlefield at the same time, does Kha'Zix still gain +2 might?

Yes, Kha'Zix, Mutating Horror will still receive the +2 Might bonus.

According to the established game rules and RiftJudge FAQs (e.g., #9849, #10198, #10465), the condition for Kha'Zix’s triggered ability ("if an enemy unit is alone here") is evaluated at the exact moment the unit enters the battlefield and gains the "Attacker" or "Defender" combat designation.

Reasoning

1. Trigger Condition Check: When you move your first unit into the battlefield, it gains the combat designation ("Attacker"). At that exact moment, the game evaluates the board state: "Is an enemy unit alone here?"
2. Condition Met: Since you only have that one unit present, the condition is satisfied. Kha'Zix's triggered ability is placed on the chain.
3. Reaction: By playing your second unit via Ambush as a [Reaction], that unit is added to the chain above Kha'Zix's trigger.
4. Resolution (LIFO - Last-In, First-Out):
   • The second unit (played via Ambush) enters the battlefield.
   • Kha'Zix's triggered ability resolves. Because the condition was met at the time the ability was triggered and placed on the chain, the subsequent change to the battlefield state (adding the second unit) does not retroactively negate the trigger.

As confirmed by RiftJudge FAQ #10198 and #10465, the condition check is not re-evaluated when the ability resolves; it only checks if the requirement was met at the moment of the trigger.