If there are two reasons for Sujud as-Sahw, it is sufficient for him to do one prostration for forgetfulness, and what appears most likely is that it should be before the salam.

It is prescribed to do Sujud As-Sahw in Nafl (supererogatory) prayers just as it is prescribed to do it in the obligatory prayers when there is a reason for doing it.

There is no specific Dhikr to be said in the two prostrations of forgetfulness (Sujud As-sahw). You can say what you say in the prostration in prayer such as (Subhan Rabbiy Al-a`la). Between the two prostrations, you can say ‘Rabbiy ighfir li’.