Can customers use discounts and store credit at the same time?

Yes, that is possible in the following two cases:

A) Shopify automatic discounts work with store credit, the discount gets applied to the cart by Shopify first, then customers will be able to checkout using their store credit.

B) For customers to add manual discount code and use their store credit at the same time, It can be done using the integration with DiscountYard app. 

Customers will be able to add their discount codes to the app discount field of DiscountYard on cart, then checkout using their store credit as normal.

Steps

1. Click here to install DiscountYard app to your store
   
2. Request free installation on DiscountYard app and the team will let you know once the installation is done.
   
3. That is it! Now both apps will work together without further configurations.

Customer will be able to add their discount codes to the cart page discount field, then checkout using store credit using the CreditsYard app widget

Note: If the cart session has an automatic discount applied while you disabled the discount from Shopify, It might get applied to the credit order as disabling the discount on Shopify doesn't clear active cart sessions unless page refresh.