The serums combining them in one bottle are the answer: they don't cancel out, and the warning is one of skincare's most durable zombie myths. Its origin is genuinely interesting — 1960s laboratory studies mixed raw niacinamide and ascorbic acid in solution and heated them, converting some niacinamide to niacin, which causes the famous transient skin flush (the flush mechanism itself is real and well characterized: https://pmc.ncbi.nlm.nih.gov/articles/PMC2779993/). But those were beaker conditions — heat, time, raw ingredients — nothing like two stabilized modern formulas meeting at skin temperature and skin pH for the seconds that matter. At cosmetic conditions the conversion is negligible, and the two ingredients are routinely and deliberately co-formulated and studied together in the same product (https://pmc.ncbi.nlm.nih.gov/articles/PMC9370691/).
So layer freely, in either order. If anything, the pairing is complementary: vitamin C works the melanin-production pathway while niacinamide works pigment transfer and barrier support — different jobs, no turf war.
The one real-world caveat has nothing to do with chemistry: tolerance. Strong L-ascorbic acid serums run acidic and can sting; niacinamide at high percentages can flush sensitive skin all on its own. If your skin is reactive and the stack feels like too much, splitting them — vitamin C in the morning, niacinamide at night — is a perfectly good comfort choice. Just know that's what it is: a comfort choice, not damage control. The "they neutralize each other" version can be retired.