Crumbling Sands

If my opponent plays two spells in succession, can I counter the first one with Crumbling Sands?

Yes.

Crumbling Sands uses the “finalize” meaning of play, because it checks whether cards have been “played” and isn’t a triggered ability. As long as an opponent (not even necessarily the one who played this spell) has chosen targets, paid costs, checked legality, and completed the process of finalizing another spell, even if that spell was countered, Crumbling Sands can counter a spell. That means it can counter the first spell played, as long as another spell was played by an opponent after that one.

Relevant Rules: 419.4.b.