if i am doing a piano duet with friend, do we get 2 certificates?

Yes you do.

The regulations say:

(j) The examiner's notes and grading will be sent to the applicant named on the entry form, and certificates will be issued for each member of a group obtaining a C grading or above.

What are you going to play?

i haven't decided yet but i am going the do the intermediate one and i want to choose Mozart Andante with 5 variations in G K.501 and of course i need to ask my friend if he wants to do that piece.