If I play Darius, and then play cleave, and Mel plays rebuttal and takes control of cleave. Would this count as 2nd card played for Darius to be readied, or how does that work since the card says take control and not counter?

No — Darius won't be readied.

If Mel pays [A], Rebuttal gains control of Cleave and it resolves as her spell, so it no longer counts as your second card played that turn (FAQ #6678). If she declines to pay, Rebuttal counters Cleave instead, and a countered card is considered never played (Rule 425.1.b). Either way, your "play your second card" condition isn't met — Darius gets neither +2 Might nor the ready, even though Cleave technically still resolves.