How do I bill multiple clients for a single project?

There are two ways of billing multiple clients for a single project:

Cost allocation – costs are divided among clients, based on the percentage you allocate to each client.
Cost allocation allows you to split project costs between multiple clients on a percentage basis. For example, you have two clients assigned to a project called Fence Installation: For Multi Trailers Inc and Excel Wheel Enterprises. Using cost allocation, you can assign 70% of the cost of the project to one of the clients, and 30% to the other client. Web TimeSheet will automatically calculate the billing amounts assigned to each that are displayed in reports.

Bucket method – costs are NOT divided among clients; the user selects which client should be billed from within their timesheet.
The bucket method allows you to create a single project, with a single project name, for similar work that you perform for multiple clients. For example, if you perform design functions for several clients, you could create a bucket method project named Design. Whenever a user performs design work, they would enter their hours against the Design project and select which client they performed the work for. Project costs are assigned to the selected client only, and are not divided among clients.

Changing Project Billing Settings:

• Select Administration from the top menu, and then select System > System Preferences from the side menu.
• Enable the appropriate billing option(s):
• Allow projects with multiple clients – Cost Allocation
• Allow projects with multiple clients – Bucket
• Click Save.
• Select Projects from the top menu.
• Click the icon for the project whose client billing you want to change.
• In the Clients section, select the new billing option.
• Add clients and, if applicable, their cost allocation percentages, in the fields that display.
• Click Save.

Note:
You typically assign clients for billing when you create a project. However, you can change a project's client billing method at any time, provided no billable hours have been entered against it.