Two bodies, requiring local space, cannot occupy the same space or any part of the same space, but must move to a different space if the first body is in the original space.
So, the body of Christ can’t be in a space in which the body of bread is locally, since both require physical space. However, if the body of bread is suddenly not bread but the body of Christ, again you only have one body occupying physical space.
But what of one body being in multiple spaces. Christ’s body is risen, and ascended to Heaven, continuing to occupy space, some space in heaven. How, can his body be said to be the Eucharistic Host? How can his body occupy space on multiple altars simultaneously around the world, and also be complete in Heaven?
If he is in heaven, is it possible that each piece of the host from the moment of consecration is somehow “in Heaven”, somehow being the body that is in Heaven?
How was it at the last supper? Jesus was holding the bread in his hands and giving it to them. So, his body was holding the bread, and his voice was heard to say this in my hands is my body. How could his body there be “two bodies”, spatially visible in two adjacent locations in space? And, also, his body holding the Host had blood in it, yet the bread did not. Moments later he took the chalice and gave it to them to drink, declaring it to be his blood, a separate Host. Indeed a sacrifice, for body and blood were not united but separated from each other as happens when a sacrifice is performed. So, what happened there?
How did he have two bodies spatially separate and two bloods, one in his veins, one in a chalice?
On the cross we know his blood drained from his body. And we also know that no one came and began consuming the flesh nor collected and drank the blood.
So, somehow the Host (sacramentally) becomes body, with a chalice of wine become blood (sacramentally) sitting next to it, just as Jesus dead on the cross was a body with blood drained from it onto the ground in a large pool (again, as happens with a sacrifice).
A person can offer another person something to eat and drink. And he names what he is giving them to eat and drink. The person eating and drinking consumes and sips the named food and beverage, believing it to be what the giver named it. The giver could tell the truth or he could lie about what he was offering. If the giver is not a liar, nor unaware of what is being offered, the receiver is indeed eating and drinking the material as named by the offerer, and not eating and drinking some other substances.
Somehow it has to be that the species, participates the other body. The bread so participates his body that it is his body, and likewise the wine. Or is he saying, “when you are looking for me, don’t look for these hands, this torso, this face and head, but here, what is given to you, find me here, for ‘This is my body’”?
In a way, this would indicate a requirement of “obedience” to him, both materially and intellectually (in the soul), turning from looking at his eyes / mouth / hands / voice to looking at the Host and Chalice.
Substance - I believe that is a term for something intelligible - Each grain of salt is “Salt”, not a part of salt but the fullness of salt. I think we do not understand the word “substance”: Philosophy calls it “The essential nature underlying phenomena, which is subject to changes and accidents”.
When we see an object, its image is in our conscious thought. Usually it all happens so quickly that we cannot dissect the process, but we recognize “what it is” so far as “substance” goes - or, rather, we associate this individual object to a “substance” we know about. But what is the process. The image : an attempt to associate that image in a category or with a substance : then output of speech or thought-“This is [SubstanceName]”. But sometimes we can find no match and so we ask -"What is it?” Someone else may declare the SubstanceName to us: “This is Yogurt”. Then, seeing it, tasting it, we understand a new substance: “Yogurt”.
If we were Jews at Jesus’ day, members of the Covenant of Sinai, and someone said to us at Passover, “This is the lamb sacrificed”, we would know what to do with it - eat it and thereby complete the sacrificial rite (it was sacrificed to God who commanded us to eat it as our part of the covenant). Its substance is not just lamb like I ate on Tuesday, but its substance is sacrificed victim that I will eat tonight at Passover. In transubstantiation, where is the change of substance? Is it in the understanding? Our “natural” association process, without the external definition of substance, says upon seeing the host and chalice, “this is bread, this is wine”. Countering this natural association comes an external claim: “This is my body / this is the cup of my blood of the new covenant.” To this, if Jesus is our King, we would rightly then think: “Oh, then I must eat it and drink, since I am in this covenant people”. Substance has to do with intelligibility. Otherwise we would be like the beasts, who treat every individual item individually, never realizing essential nature. The beast at most remembers this appearance is desirable, tasty; this appearance is not desirable, not tasty. Only the human decides to eat something because of its understood substance.
At Mass, if we eat the substance identified by our reason (if we eat “bread and wine”), we are setting our reason above the One who says, “This is my body / blood”. One place that transubstantiation happens is in the soul - the understanding of “what is this” (the “whatness”) we are eating and drinking. As we approach the priest, we might think: “What is this you are about to hand me to eat?” and the priest is then heard to say, “The body of Christ”, and then we say in response, “Amen (, so it is, so I shall eat)”.