If I play Darius second, will he get his effect?

Yes, if you play Darius, Trifarian as your second card in a turn, he will trigger his effect and receive +2 Might and ready himself.

According to the RiftJudge FAQs regarding Darius, Trifarian:

• Darius counts himself as a played card. In Riftbound, a card is considered "played" once it has fully resolved and entered the board. Because Darius is already on the board when his "When you play your second card" ability checks, he sees himself as the second card played.
• Sequence:
  1. Play your first card.
  2. Play Darius as your second card.
  3. Darius resolves and enters the board.
  4. His ability triggers, giving him +2 Might and readying him.

Important Nuances:

• Darius must be the second card: If he is played as the third, fourth, or later card, his ability will not trigger because the "second card" condition has already passed.
• Tokens do not count: Playing a token does not count toward the "second card" condition. Darius must be the second actual card played in the turn.
• Already on board: If Darius is already on the board from a previous turn, he will also trigger his ability when you play your second card of the current turn.

Source: RiftJudge FAQ #7647, #1119, #4088, #8290, #7994, #7847, #6776, #7242, #5141, #6155.