At this time, Shopify doesn’t currently offer “stackable” discounts in that, only one discount code can be used per order. That said, I know this is a wanted feature by many and something I’d love to see added to the platform. For this, I’ve gone ahead and added your vote to the existing feature request to show our teams there is further interest in implementing this in a future update.
In the meantime, if you’re subscribed to the Shopify plan, then you could consider offering your customers a gift card with the balance of the discount code you wish to offer them. Alternatively, you could partially refund their orders or use apps like Locksmith to lock down specific marked down products so that only your preferred customers can access the discounted prices.
If there’s anything else I can help with please don’t hesitate to reply back and I’ll happily help out!