In short: Add one Product Discount per collection, each with its own percentage and Specific Collections target. One code then gives each collection its own discount.
Why one code: a Shopify discount has one value, so 20% off one collection and 10% off another would be two codes. OneCode: Stackable Discounts does it with one.
Example
Code SUMMER: 20% off the Summer Collection and 10% off Accessories.
Steps
Click Create discount, choose Discount code and enter
SUMMER.Click Add Discount Type. Enter
20in Percentage, set Target to Specific Collections and pick Summer Collection.Click Add Discount Type again. Enter
10, set Target to Specific Collections and pick Accessories.Click Save discount.

What the customer gets
A $50 summer shirt and a $20 tote from Accessories:
Shirt: 20% off, $10 saved.
Tote: 10% off, $2 saved.
Good to know
If a product is in both collections, it gets the bigger discount (20%), not both.
Add as many collections as you like, each as its own discount type.
Add free shipping to the same code with a Shipping Discount.
