if the first spell got defied, can I still use crumble sand to counter the second spell they played

You're asking about Crumbling Sands (not "Crumble Sand"). Its text is: [Reaction] Counter a spell if an opponent has played another spell this turn.

The answer depends on when you play it, because a countered spell does not count as "played" (Rule 425.1.b).

If you play Crumbling Sands while Spell 2 is on the chain, before Defy resolves — yes, you can. Spell 1 is still on the chain as a Finalized Chain Item and counts as "played" at that point (Rule 419.4.b). The condition "an opponent has played another spell this turn" is satisfied, and Crumbling Sands can target Spell 2.

If Defy has already resolved and countered Spell 1, then you play Crumbling Sands — no, you cannot. Per Rule 425.1.b, Spell 1 is not considered "played" once it was countered. So when you check the condition, the only spell your opponent has "played" this turn is Spell 2 — and there is no "another spell" to satisfy the requirement. The condition fails and Crumbling Sands cannot be played.

Timing summary: Play Crumbling Sands before Defy resolves (while Spell 1 is still on the chain), not after.