Is it permissible for a man offering a Sunnah prayer to lead another offering an obligatory one?

There is a consensus among Muslim jurists that the rawatib [Sunnah prayers associated with prescribed prayers] are not audible. Based on the opinion of Muslim jurists who permit it, if a person who is performing an obligatory prayer follows another who is performing a Sunnah prayer, the latter must complete his prayer as usual and recite in a low voice, except when he makes takbir [saying Allahu Akbar] which indicates a movement from one position to another e.g. from ruku’ [bowing] to sujud [prostration] etc. Imam Al-Nawawi (May God have mercy on him) said in Al-Majmu’: "The supererogatory prayers performed before or after the obligatory prayers are not to be performed audibly.”
And God the Almighty knows best.