Discount codes can't be stacked, so your customers would need to choose one or the other.
However; if you want to have the discount appearing on each product, wherein the old price is crossed out, and the "sale" then you'll need to edit your 15% welcome discount to exclude the sale products. You can find more information on how to create and edit discount codes here.
Alternatively, you can use a discount app to help you manage the discounts.
Trevor | Community Moderator @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog