For dual-manifest missions, Ariane 5 often uses the Système de Lancement Double or SYLDA system to encapsulate the lower of two stacked satellites. It can be seen in the following image encapsulating the Planck spacecraft.
This fairing is separated after the first satellite, placed above, has been separated. Therefore, it separates in whatever the target orbit is.
How long does it take for this fairing to reenter? It seems like it might take a very, very long time.