Feature added in Pipeline 6.1
Pipeline can be customized to match any product with another when using metafields. This allows you to customize which product is displayed as the "But it with" promotion on each individual product page.
There are three initial steps involved when setting up custom paired products:
Create an upsell product reference metafield called theme.upsell
Go to each product that should have an upsell and select the product you want to upsell
Connect the metafield in the Theme Editor using the "upsell" block on the product form
After the initial setup, each product can easily be paired with other products using your Shopify Admin under Product setup.
Follow the steps in the Cart upselling article to learn. When you use an upsell metafield called theme.upsell, the "Buy it with" promotion appears on both the Product page and the Cart: