At the beginning of a case that involves multiple payers, create a client and a case as usual. Then create a client records (Add Client button) for the second payer (system supports any number of additional payers).
Then under the Cases (Projects) tab, select the newly created case and click on Edit Case on the right. In that screen, select the second payer in the dropdown and enter what percentage of bills this person is responsible for.
If both payers has given retainers, click on the Accounts tab, select your trust account and click on Add Funds button. On the popup select the first payer and enter amount. Repeat the same process for the second payer.
After this setup, do the timekeeping in the case agnostic to the case being a multi-payer case. Generate invoices as usual. When payments are recorded against invoices of multi-payer cases, the system will ask who the payer is making that payment and will keep track of payments from different payers. On the payment record popup, payments can be made from selected payers trust funds.