When you check out an appointment on your Schedule or sell items from your Cash Register, you are generating an invoice that is tied to a specific client's unique record. For this reason, you can check out multiple services on a single appointment for the same client, or sell multiple products or services for the same client, but you cannot generate accounting activity for two different clients in one transaction.
If you have one client who is paying for the service of another client, it is best to ring the transactions up separately for each client. If the client is paying by credit card, the card will need to be charged for each transaction. If the client is paying by check, simply record the same check number during both transactions.