Not sure if we should bother that much with lore explanations for multiple trusts.
In theory, lore-wise, Trust is a new type of magic that creates a "copy" of someone when a Link is estabilished through the bond between caster and target.
This is the first inconsistency because it doesn't explain what happens with Ciphers.
But let's carry on.
Being trusts "copies" there can be hundreds of them and, as a matter of fact, you can see many of the same trust in the same spot if multiple players summon it.
Game doesn't explain why you shouldn't be able to summon more copies of the same trust, so really... why are we bothering to find a fitting explanation for all of this lol?