In Secret Misson Risk which player is the winner when 2 missions are simultaneously achieved?
From these rules:


  The player who completes his or her mission first-and reveals the Mission
  card to prove it-wins.


The rules make no mention of ordering, or of only being able to win at a specific time. If both players achieve a mission at exactly the same time, and both reveal their completed missions, then they have both fulfilled the win condition. You could assign a single winner in any arbitrary manner, but personally I'd say they both win; after all, does victory have to be exclusive?

Extra note: regarding the timing of defeating the final army and capturing a territory, the rules say:


  As soon as you defeat the last opposing army on a territory, you capture that territory and must occupy it immediately.


While the rules don't specifically set out timing, I'd say that the "as soon as" and "immediately" mean that capturing a territory happens at the same moment as defeating the last army.