It's not possible to split a payment for one transaction over Afterpay and any other payment method.
If the client needs to split a payment with Afterpay, then you would need to place two separate transactions. One to be paid for via Afterpay, and the other to be paid for by whatever other payment option the client wishes to use (e.g. EFTPOS or cash). Remember that there is a minimum $50 spend for an Afterpay transaction.