If you are using the Shopify Checkout Integration customers can apply multiple discount codes at checkout, however, once those checkout orders are processed Recharge will only apply one discount code to the customer's recurring orders.

Customers can only apply a single code per order. It is not possible to stack discount codes. For example, if the customer has a 10% off discount code applied to their cart, the customer cannot also use a "save $10" discount code on top of the existing code. To offer customers multiple discount codes, create a discount code for the value of the combined discounts.