Every transaction will have the internal ID for the unique identification of transaction record, when you create one Invoice this means that a payment record for the same customer must be applied which will also contain another unique identification of transaction record.
An Invoice will also have the Billing and Shipping address where the orders should be shipped, including the emails where the Invoice must be sent.
Creation of Invoice with multiple customer is not possible as the Payment that will be created should have the same customer. Additionally, creation of Payment for multiple customer is also not possible.