Multiple assignments with different work schedules: overtime and schedule header on time card

Summary:
An employee has two assignments, each with its own work schedule:
Assignment 1 : Mon - Fri, 08:00 - 17:00
Assignment 2 : Mon - Fri, 12:00 - 21:00

When the employee selects Assignment 2 on the time card, how can we ensure that:

1. Overtime is calculated based on Assignment 2's schedule (12:00 - 21:00) rather than the schedule associated with the primary assignment?
2. The schedule shown on the time card header reflects the schedule for the selected assignment, rather than the primary assignment's schedule?

For example, if the employee records time against Assignment 2, we would expect the system to use 12:00 - 21:00 when determining overtime and to display the corresponding scheduled hours on the time card.