Currently, an Invoice can only be Invoiced to one Contact or Account. When creating an Invoice, you will have the opportunity to select 'Billed To"

If you wish to split an Invoice, you can create multiple Invoices & issue them to different Contacts or Accounts.

Prepayment Invoices can be used to create invoices with a partial percentage. You can use this beta feature to Invoice one client 50%, and the other client the remaining 50% (or whichever percentages are suitable).