Twin Strike is actually two attacks:
Attack: Strength vs. AC (melee; main weapon and off-hand weapon) or Dexterity vs. AC (ranged), two attacks
Which means that a critical hit on either of them would be eligible to trigger Epic Resurgence. Contrast that with Sweeping Whirlwind, for example, which makes a single attack against multiple foes. Only the if the first attack roll you make is a critical can you take advantage of Epic Resurgence.
The intent is to limit the ability to trigger the feat by making area attacks that include large numbers of opponents using one action; powers such as Twin Strike that let you make multiple distinct attacks as one action are typically limited to two or three attacks at most. At the extreme other end, consider something like Cloudkill, which as an Area Burst 5 could make at least one critical hit remarkably likely.