The phrase "Christ is in two natures," or worse "Christ exists in two natures," makes no sense to me. Unless the natures are subsistent. This is the (unintended?) implication of the phrase.
The result was that Nestorius praised the Tome of Leo because it said "in" two natures. In fact most of the original defenders of Chalcedon (prior to around 518) were pseudoNestorians (who rejected the 12 Chps) who interpreted Chalcedon to mean subsistent natures. So it does have theological significance.
Well, so long as we would describe Christ in any terminology invented by man, it will never be a perfect way to describe Christ.
Now, using the issue that Nestorius supported the phrase "in two natures" doesn't necessarily make "in two natures" heretical. That's like saying Apollinarius and Eutyches used "one nature" or "of two natures" so much so as to "blend" those natures together, at least the other side of the argument is saying.
If anything also, while "pseudoNestorians" existed, we can't say that all of Chalcedon were pseudoNestorians. We can say that there were Orthodox members who indeed interpreted it differently from those pseudoNestorians, and certainly as Orthodox believe, this cannot be separated from the fifth council, clarifying Chalcedon's intentions.
So, now the theological significance may be in fact helpful if we combine both traditions. How can one understand that Christ is "in" two natures? Well in theory, if we talk about the divinity, we see Christ "in" it, and if we talk about humanity, we also see Christ "in" it. But doing so alone obviously is to make distinct the natures of Christ to the point of separating the natures, and therefore, we must also emphasize the unity of Christ who is "of" two natures, or "one Incarnate nature," so as to show how the humanity and divinity of Christ act together not as two separate and side-by-side entities, but as one under one person, who is Christ, the Logos of God. And if were to emphasize only "of two natures," the unity can be misconstrued as indeed a blending of the natures, or the loss of one for the sake of the other.
As we agreed with one another:
Ever since the fifth century, we have used different formulae to confess our common faith in the One Lord Jesus Christ, perfect God and perfect Man. Some of us affirm two natures, wills and energies hypostatically united in the One Lord Jesus Christ. Some of us affirm one united divine-human nature, will and energy in the same Christ. But both sides speak of a union without confusion, without change, without division, without separation. The four adverbs belong to our common tradition. Both affirm the dynamic permanence of the God-head and the Manhood, with all their natural properties and faculties, in the one Christ. Those who speak in terms of "two" do not thereby divide or separate. Those who speak in terms of "one" do not thereby commingle or confuse.Bristol 1967
Now, if you want to debate the specifics, there's always the private forums. But if you're confused about terminology, then we shouldn't even mention history.