Please view the PayFast Developer Documentation for information on how to integrate Split Payments into your platform.
The primary account holder will have to enable Split Payments in their dashboard and then provide you with their Merchant ID and Merchant Key, which has to be included in the string.
You are required to work out with the merchant directly what the payment split to your secondary account should be, that is either a fixed amount, a percentage or a combination of both. A minimum and a maximum value can also be set.