If you apply two sales to the same product, Bold Discounts will use the discount applied first, and skip any other discounts. This prevents a product from being double discounted by mistake.
Discounts does this by tagging each item that it places on sale with hidden "meta-fields". Any product that has Discounts' meta-fields attached to it will be skipped from being discounted. Visit Metafields (links to Shopify) for more information on this.