AHA + BHA

Be a bit careful. Two exfoliating acids at once can over-exfoliate - glycolic plus salicylic in one session is where tight, shiny, stingy skin comes from.

The default: alternate days. AHA for surface texture and dullness, BHA for inside-the-pore clearing - each gets its own day and both work better than stacked.

The exception: products formulated to combine them at sensible strengths, or skin that's well used to acids. If a single product holds both, the formulator did the math; if you're layering two strong separate products, you do the scheduling.

Over-exfoliated already? Stop all acids, moisturize, and let your barrier rebuild for a week or two. Then come back slower, one acid at a time.