Is it not possible to add two discounts per product to the same line item? I’ve been having this problem recently.
Search first https://community.shopify.com/search?q=+apply+multiple+discounts+to+the+same+product
Turn on discount combinations.
https://help.shopify.com/en/manual/discounts/discount-combinations
Otherwise In what ways, context matters a lot if your going past what’s clear in the docs
eg. is this ajax related? who knows you haven’t provided any replicatable context https://shopify.dev/docs/api/ajax/reference/cart#update-discounts-in-the-cart
Generally just use apps for this, or the draft order system e.g. https://tasks.mechanic.dev/create-a-draft-order-from-the-cart
If your a dev and this is api related then use the dev forums htttps://community.shopify.dev