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

I don't find a card named "Crumble Sand" in Riftbound — it doesn't appear to exist in the game. You may be thinking of another TCG or misremembering the card name.

That said, I can answer the core chain question:

Yes, a Defy that countered the first spell does not stop you from countering a later spell.

The chain resolves one item at a time (LIFO), and after each item resolves, a priority window opens. So the sequence works like this:

1. Player A plays Spell 1. Chain: [Spell 1]
2. You play Defy targeting Spell 1. Chain: [Spell 1] → [Defy]
3. Defy resolves — Spell 1 is countered and cleared. Chain is now empty.
4. Priority window opens. Player A plays Spell 2. Chain: [Spell 2]
5. You can now play your own Reaction counter spell (whatever it is) targeting Spell 2.

Each spell is an independent chain item, and countering one doesn't affect your ability to counter another later. You just need a valid target on the chain at the moment you play your counter spell — and after Defy resolves and Spell 2 is played, Spell 2 is a valid target.

If you can double-check the exact name of the card you're thinking of, I can give you a more precise ruling.