The best way to handle this is to create two new call types (Admin>Scheduling>DDM-Scheduling>Service Types):
Couple Care (billable)
Couple Care (non-billable)

Once you have created the call types, you'll want to configure the non-billable one to Not billable and Not payable, and check the box to make them both not calculate OT pay (Admin>Scheduling>Service Types).

Once the call types have been created, you can set the rates for them on the rates tab in your office settings.

The last step is to create the schedules in the calendar, using one Service type for one person and the other Service type for the other.