What do you call it when something's worth is two things at the same time?

He taught him humility. Doesn't all this religious stuff boil down to that? But in terms of two opposites: the lesson of things not always being what they seem.

That is, reality is paradoxical, a paradox.

two opposite situations: worthless dessert/great spiritual significance

Paradox: Merriam Webster:

c: an argument that apparently derives self-contradictory conclusions by valid deduction from acceptable premises

this lesson on paradox

Ambivalence: the state of having two opposing feelings at the same time, or being uncertain about how you feel (dictionary.cambridge.org)

My gut feeling is that this word doesn't fit in your example as well as the one suggested by Elliot, but I have to include it because its etymology so perfectly fits your question.

The above definition describes a state of the beholder in whose eye beauty so famously resides. A deeper discussion of value is beyond this post, and Elliot addressed it well already; suffice it to say that the true value of something could be argued to be independent of someone's perception of its value. One place to start that discussion is at https://www.britannica.com/topic/gold-standard.

OP's comment showing the other side of the coin should be saved in relation to this discussion, too, so I quote it here just in case: "What is salt worth or spices worth. It all depends on supply and demand. it depends what you are going to use it for."[sic] --greay

An entry at etymonline says it comes through German, contrary to what I previously assumed: that it came directly from Latin "two values":

"simultaneous conflicting feelings," 1924 (1912 as ambivalency), from German Ambivalenz, coined 1910 by Swiss psychologist Eugen Bleuler on model of German Equivalenz "equivalence," etc., from Latin ambi- "both, on both sides" (see ambi-) + valentia "strength," abstract noun from present participle of valere "be strong" (from PIE root *wal- "to be strong"). A psychological term that by 1929 had taken on a broader literary and general sense.

So you could say that Saladin sees "two strengths" of Jerusalem.

Related: Is there a single word for "Evoking Ambivalence"? (special mention: both OP and Patrick's answer)

Worth two things at the same time is a Duality. See bottom paragraph. In this case Duality of value.

Perspective is a good word that may describe Saladin's view but not his experience. It must have been near the climax of the film to ask such a profound question. The lesson must be profound or you have wasted your ninety minutes. If we can't entertain words for such profundities we can't very well boast of our literacy.

By "Nothing and everything." he is telling us his experience of Jerusalem. The two extremes coming together make an important duality, such as the definition of God as 'an infinite sphere, whose centre is everywhere and whose circumference is nowhere'.

In this case his experience has been seeing friends struggle and die for the advantage of a few extra square feet of territory that's just like any patch of dirt one hundred miles in any direction. Like so many things of purported value it may not turn out to be worth what it actually cost you. You find that out after you see what you can really do with it once you've got it.

I think the right term would start from the original question of "What is it worth?" The answer should be Value. The meaning behind the answer, Value, would be as loaded and open to interpretation as any other you could think of. It would remind the victory minded of their great glory and the more sober minded of their terrible cost. The range of these answers bring forth the duality.

Your thoughts are interesting but the word juxtaposition makes me think of the meaning switching from one to another; only one at a time. What Jerusalem means is both and at the very same time. Both the worthless desert and its great spiritual significance to religion. The duality is vital to its true value.