Incidentally, the ever-dependable Zadok has answered
about Pope Benedict's distinct installation ceremonies and hence, has proven to be more intelligent than me.
Zadok points us to Can. 332 §1: The Roman Pontiff obtains full and supreme power in the Church by his acceptance of legitimate election together with episcopal consecration (emphasis added).
[M]y understanding is that both acts are purely symbolic and relate to two intimately related aspects of the Papacy - the Pope as Universal Pastor and the Pope as Bishop of Rome. This second ceremony whereby he takes possesion of the Lateran is symbolic of his role as Ordinary of the Diocese of Rome. However, it should be noted that (to the best of my knowledge) neither ceremony has any jurdical effect. (This in in contrast with the fact that other diocesan bishops have to 'take canonical possession' of their diocese before their exercise their power to govern.)
I concur. My concern was that, if both ceremonies were required to grant canonical jurisdiction - first, respective to the universal church, and second with respect to the Roman - this would create an extraneous and arbitrary separation between the two offices, which I believe to be conjoined. If, as Zadok puts it, both ceremonies are purely symbolic, and if the Pope receives full jurisdictional power the moment of his election, then my concern is resolved. (Not that the Roman curia were waiting on the edges of their seats for me to think this through, but it does help me...) It may be best to think of them, as Zadok does, as two aspects of the same office, rather than two separate offices.