You can apply discount codes and automatic discounts to your subscription products. You'll need to configure the purchase type within the discount to allow the discount to be applied to subscriptions.
These discounts will be added alongside the discount you have configured in the Selling Plan Group.
Edit purchase type on discount codes and automatic discounts
Go to Discounts in the left sidebar in your account
Select the discount which you'd like to edit the purchase type
Look for the Discount Value section > Purchase Option
In Purchase Option select either both or subscriptions, depending on if you want both one time purchase and subscription products to have this discount, or only subscription products.
In Applies to, choose the products to which you'd like this discount to apply.
Save
Now this discount and the discount you have set up in your Selling Plan Group will apply to the products you choose.