First solution is not to use all capital letters when writing a topic. This looks rude on internet and decreases responsiveness of people
Second solution is to go with pH 6. Neutralize salicylic acid with NaOH to get Sodium Salicylate. There are some studies showing it can work as great as Salicylic Acid in the long run, but with less irritation and suggested for sensitive skin (see https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1468-2494.2009.00501_2.x)
Third solution is to go with pH 3.5. Many claim that Niacinamide is not affected by acidic pH level, but personally I prefer to avoid acidic pH for Niacinamide.
Fourth solution would be to make anhydrous formula. No water - no pH. So mix of Glycerine and propylene glycol allows to dissolve both Salicylic Acid and Niacinamide. No preservation needed too.