So theres this girl, and shes born under the destiny of destroying the world.
And theres this guy, and he cares about the world being destroyed, but he cares more about the girl being killed, which sucks arse because thats what hes destined to do.

To fully understand the technicalities of this story, one must first understand the fundamentals of reincarnation. Time has no say in recarnation. You may be reborn before you were born, after you died, or during your life time.
Now this girl, named Destry for the duration.
Reincarnated.
At the time of her birth, lived with her reincarnate, posing as a twin.

Following the logic that the number of living things must be finite (if redundantly large), all that would be nessecary would be one, single, soul destroying suscide, right? If every single soul rotation accumulates more and more touched souls, a single death would be sufficient for all. Translates to; Destry must die. Depending on which one of her live, her or the incarnate, could annihalate the world, or just simply ebb away, and cycle.

Hard to understand? Difficult to piece togther? This is the kind of stuff I write.
Tell me what you think, or at least tell me what you can't understand.

My biggest concern is that it's her destiny to die/destroy the world. Be careful with the concept. It's been done so many times before that you're really going to need to make these character's loveable for the story to fly. You're also going to need to give us a really good reason Destry's soul is chosen for this.
I don't mean to say that it can't be done. Clichés are clichés because they work. It's just the way that you present it to your audience. That's what you need to be careful with.

I like your take on reincarnation. When the story/plot gets more finalized you might consider changing the name of it. I only suggest that since "re" means to do again, and if this version of reincarnation can be done before the birth of the "original", it's not really being done again.
Like I said, I really like the concept, I just think it might be served better under a different name.

Now, when you say "posing as her twin" this seems to imply that the reincarnate knows that she's a reincarnate. Is that the case? If so, how? And how does the Destry not know?

You kind of lost me in the last paragraph. I understand the idea that all living things are at a finite number, but what do you mean by "necessary"? Necessary to destroy the world/throw it out of balance?
If that's the case, then I agree.

But I don't understand is that why it would matter whether Destry or her incarnate dies. If they're part of the same soul/are the same soul then wouldn't destroying the soul of either of them cause both of them to die? Or are you operating under the idea that Destry is the "main" soul and that the death of the incarnate’s soul is just a "copy", which can be destroyed without issue?

The details about reincarnation and the finite and precariously balanced number of living souls is a very interesting idea and I quite like it. I hope you can make this story work out the way you want it to!

It is a little confusing in some spots that takes a few read throughs to grasp completely, so my main advice would be to make sure that you're very clear when explaining certain points of the story, in the /actual/ story. If a person reading your actual work gets confused too much, it loses the interesting factor and becomes something they have to decipher: which isn't fun. (You also have to keep your target audience in mind throughout.)

That said, I find the idea very interesting. As was already mentioned, you have to be careful with ideas that have been done before: while some people will love it for having it's own twist on an idea they love, others will tear it down endlessly. You have to be sure that loads of things in it are unique to your story, that you are making the idea your own. The reincarnation bit sounds like it's a great start to making it a bit more unique and interesting.

The last paragraph also confused me, the general concept.

a single death would be sufficient for all. Translates to; Destry must die. Depending on which one of her live, her or the incarnate, could annihalate the world, or just simply ebb away, and cycle.

If this is saying that if Destry dies, the incarnate will as well, how does it matter which one is the one to die? If this is something better explained in the actual story, then that's fine; if it's not, you probably want to clarify that point there.

Generally, though? I'd probably read it. It would likely be one of those things, because of my own personal preference, that I would shy away from reading until finally someone told me I needed to check it out; and then when I did, I'd love it.