You can add multiple customers to an aircraft and bill them separately on the same work order
Adding Multiple Customers
Add each customer separately in the Customers module and add their contact information.
Adding the Aircraft
Once you have added the aircraft, when viewing the Aircraft, go to the Customers sidebar and add each customer.
Note that you can select the primary customer for the aircraft by entering the customer in the Aircraft's Main Info sidebar > "Customer" field (near the very top).
If you have multiple cities, you can enter the default city: this is useful for reporting, but you can always create a work order under any city.
Creating a Work Order
Create a work order for the aircraft. The work order will default each item to the primary customer.
Billing a Different Customer
You can change the item to bill to the other customer(s) from the Items sidebar > Item Details tab > Billing tab: change the "Billing Customer".
You an also enable "Allocation" billing, allowing you to split an invoice not by item, but by specific amounts. This is an "All Cities" option.
Once this is set, you can set the allocation amounts in the Work Order > Config & Billing sidebar > Allocation tab.
Printing an Invoice
By default, the Reports > Customer Invoice will print the primary customer's invoice. You can select a different customer by selecting Customer Invoice (With Options) and select Customer and select the non-primary customer.