Complex sentence: There are two "pudrideros" at El Escorial, one for the Pantheon of the Kings and the other for that of the Princes, which can only be visited by monks from the Monastery.
Split into simpler sentences: There are two "pudrideros" at El Escorial, one for the Pantheon of the Kings and the other for the Pantheon of the Princes. These can only be visited by monks from the Monastery.