Hi Soumya,
Firstly thanks for your time and effort 
Yeah I did go through KBA 2188042 before. Its actually talking about giving multiple interest rates to different due items of a customer but I need two different interest to be calculated for a single due item. Correct me if I'm wrong.
Its not fixed amount Soumya it varies with the invoice amount so I wont be able to use the fixed amount of interest calculation. Even I thought of iusing the custom FM to calculate interest and currently working on it. I haven't checked the INT_Formular will take a look into it and keep you posted.
Thanks a lot Ma'am!
Regards,
Vinay