See below (from the Help menu for GL Labor Posting); note the last sentence indicates that any cross charges for labor transactions are handled via inter-company processing. You would need to establish the Due to/From relationships in the Multi Company module.
Post Company {company ID} (button)
When you click Post Company, a process starts that posts labor transactions for the specified fiscal period to General Ledger. Transactions for all employees whose home company matches that of the user’s login are selected for processing. In addition, if the employee’s home company does not match the company of the labor transaction, inter-company postings are created in General Ledger.