I am not a Catholic so I may not provide the full or best answer but from the Catechism of the Catholic Church, as far as I can tell, implies that these 'four distinct layers' are potential there for any scripture. In the Protestant reformation there was a clear 'break away' from this method. So it is certainly not the case of your second option 'each passage to have two senses', rather the three kinds of spiritual senses are all potentially existing at the same time under the Catholic model.
Here is an example of how these 'four layers' are worked out in Medieval Christianity. John Cassian, a 5th-century monk, applied the four layers of Jerusalem as:
"according to history (secundum historiam) the city of the Jews, according to allegory (secundum allegoriam) the Church of Christ, according to anagogy (secundum anagogen) that celestial city of God, which is the mother of us all, according to tropology (secundum tropologiam) the human soul (source here).
A brief summary of the Catholic four layers can be found here, Ancient Faith for the Church's Future, if you are still interested.
We are definitely taking here about four Catholic Traditional layers (not two). This four layer system is not part of the Protestant Tradition, and so more or less opposed by most Protestant Bible Colleges. Yet as usual the waters are always murky on such things as the article I linked to is actually an InterVarsity Press publication, which seems to be an Evangelical Publication based on their statement of faith. In this IVP publication the author is kind of promoting a return to a “layer like-ish’ view but more along typical evangelical lines. However, I do not know if many Catholics believe that these layers are always there, or just potentially there. Maybe the answer is not that important because once you say 'always potentially there' as in the Catechism, its juts a matter of finding them.
Possibly what is causing the confusion and so much discussion recently on this topic is that the term SP frequently used among our members is not really in accordance with any tradition, Jewish, Catholic, or Protestant. The discussion has been based on a proposed 'heaven' and 'earth' four layer combination (HH, HE, EE, EH) which is not to be found in any historical book, Jewish, Catholic or Protestant. 'Sensus plenior’ as an academic real term is basically a Catholic asserted Latin phrase and the meaning of 'sensus plenior' according to that definition is explained here sensus plenior.