Solution:
The following link shows how to make parallel payments with Express Checkout: https://developer.paypal.com/webapps/developer/docs/classic/express-checkout/ht_ec-parallelPayments/
It says while parallel payments follow the normal Express Checkout flow, they differ in the following ways:
- The SetExpressCheckout request includes different items offered by multiple merchants.
- The SetExpressCheckout request contains primary information, as well as optional secondary information that adds detail to the items being purchased.
- To capture the payment, your DoExpressCheckoutPayment request must itemize the different merchants and payment amounts.