Or you do one(fardh)or the other(sunnah)?
Also, to become a Muslim,is it necessary to declare that you are a Muslim in the company of two other Muslims?
Fardh and sunnah prayers have to be performed separately.
You don't need witnesses to become a Muslim. Once you utter the Shahadatain believing in them, you are then a Muslim.
Please see the section on "Accepting Islam & Related Issues ".
Sheikh Muhammed Salih Al-Munajjid