Is there anyone who uses both niacin and vitamin C products?

I am currently using Manyo Factory Galactomyces Niacin Essence. I bought and am using it because it contains galactomyces ferment and niacin, which inhibits melanin, and is said to provide tone-up, whitening, and radiance effects. I do my skincare routine every morning. I am satisfied with it because it is moisturizing, and I feel like my skin has gotten a little brighter.

However, when I look for whitening products, I often see mentions of Vitamin C. Niacin and Vitamin C? It seems like both are ingredients effective for whitening.

1. If any of you use products with two different ingredients separately on the same day, could you please explain how you do it?

2. I would like to know if anyone uses a product that combines both ingredients into a single bottle. I prefer to use products that provide combined care rather than just separate ingredients.