First. You can create multi contract and put the main contract only in run state 

Second you need to make custom in payslip by doublicating contract field 

Then use paython code. With. Object. Contract. And new contract2. To calculate salary

You can implement this in Odoo 10 like so:

1. Define multiple contracts for the employees concerned. For example, for a particular employee you might have a "NORMAL" hourly wage of $12/hour and a "SPECIAL" hourly wage of $18/hour (e.g. overtime hours).

2. Create a salary rule for normal hours. The python code will be something like worked_days.NORMAL.contract_id.wage * worked_days.NORMAL.number_of_hours

3. Create a salary rule for special hours. The python code will be something like worked_days.SPECIAL.contract_id.wage * worked_days.SPECIAL.number_of_hours

4. You can optionally have one salary rule incorporating (adding) the two relations above, depending on whether you want to display the normal and special pay separately on the payslip or not.

5. Create a salary structure incorporating the rules above and other desired rules.

6. While creating the payslip, create two inputs under Worked Days like so (a) an input with code "NORMAL" linked to the normal contract and (6) another input with code "SPECIAL" linked to the special contract. On the payslip, be sure to select a Contract (any of the ones defined for the employee) and the right Salary Structure, to get the right calculations.

7. Check that everything else is fine and click on "Compute Sheet".