https://www.doctorofcredit.com/paypal-now-allows-splitting-payments-between-two-cards-select-merchants/

Looks like it may only be for select websites right now. I don't see the option every time.

You would just set up multiple payment cards in your PayPal account, then when you go to pay on the site, choose "PayPal" at checkout. It would then redirect you to your PayPal account, where you'd see the option to "split payment." You specify how you want it split there, and when you're returned to the Pendleton website, it just shows "PayPal" as the payment method. Pendleton doesn't know anything about how the payment is being handled by PayPal.

Also, if they have PayPal as an option for checking out, that has a "split payment" option. Not sure if it would achieve what you're looking for.