Strict Matched-Pair Cross-Sell Discount

- Merchant wants to nudge customers to add a second, specific SKU once a first SKU (from a defined pair, A or B) is added to cart — works in either direction (A→B or B→A). - Once both SKUs are in the cart, a % discount applies, but ONLY to matched/equal quantities of A and B (e.g. 3x A + 2x B = discount on 2A + 2B only; the extra A stays full price). - Discount must be strictly scoped to the A+B pair — if a third SKU (C) is in the cart, even if C is part of the same campaign, it should NOT receive the discount. - If the second SKU is never added, checkout proceeds normally with no discount applied.