Accessing rate values associated with two diff rate types in Payroll

Summary

Need to fetch rate values defined in grade ladder with two diff Rate types - Salary and Staff Assessment

Content

Dear Team,

Context: 

We have created - Grade Ladder: UN International Staff Rates
Grade Type: Grade with steps
Grade Step Rates & Step rate values: 
1. Salary International ( Rate type - Salary) 
2. Staff Assessment International ( Rate type - Staff Assessment) 

Salary Basis : UN International Salary Basis

we were able to associate either Salary or Staff Assessment to the element through the salary basis.  So we associated "Salary International" to the payroll element. It works as expected. 

Solution Needed: 

As we can associate only one salary basis to the staff, we are not able to associate the "Staff Assessment International ( Rate type - Staff Assessment)" directly or indirectly to the payroll element.