At the beginning of a case that involves multiple payers, create a client and a case as usual. Then under the Cases (Projects) tab, select the newly created case and click on Edit Case on the right. In that screen, enter information about the second payer (and also fee split percentage).

(this is a one time process for each multi payer case) Now add a time entry in this case. In the time entry box, enter 0 in the total cost box. Save. Now click on the Invoices tab. Create an invoice for this 0 dollar entry by clicking on Create Invoice button. After invoice is generated (it will be found under Fully Paid and Written Off invoices sub-tab inside the Invoices tab), click on the green dollar icon in the invoice listing (inside the Invoices tab). Enter the first retainer payment received from the first payer. In the description, enter payer’s first and last name (these should match the client name or additional payer name of this case). Save the payment. Now click on the green dollar icon again and record the retainer payment from the other payer (if the other payer also put up a retainer payment). Enter the payer’s first and last names in the description box. Save.  

From now on, do the timekeeping as usual, generate invoices and enter payments (make sure to enter payee's first and last names in the description box). System will keep track of retainers and payments individually.