Set the Rate code type for both of the overtime rates as Overtime Rate (hours):
On the assignment - main tab select the Detailed - Allow Candidates to enter detailed units of their working hours.
Set up the costing rates as below selecting the rate rule as Candidate Entered and entering a different rate code if required for the week day overtime and the weekend hours:
The candidate timesheet entry screen below, will allow the candidate to enter their working hours and then the time breakdown.
For a week day they will select both one day and the overtime worked.
For the weekend they can enter just the hours worked: