How to bundle subscription products (without the Bundles app)?

[worldmatt]

OK, this is fun: We want to offer our customers a way to bundle products and save some money. Easy, right? Not so fast!

1. The discount would be applied if a customer chooses at least 1 product from Collection A and 1 product from Collection B.
2. But! All the products are subscription products, with prepaid/gift subscription plans in Recharge. They're delivered on different cadences, Collection A every 2 months, Collection B every 3 months.
3. They're also fulfilled by different vendors. We fulfill Collection A ourselves, while Collection B is fulfilled by a close partner.

The Shopify Bundles app doesn't work, because it creates a new, single product that, in Recharge, would have to be on a single subscription plan—one that can't account for the different shipping cadences.

I'm playing around with the standard Discounts system, but I don't see a way to make the discount automatic if products from the 2 collections are in the cart.

Ideally, we'd love to have a system where if you Add to Cart a product from Collection A, you get a reminder (somewhere, somehow) that if you buy a product from Collection B, you'll get a discount on both.

How should I get this done?