Both timelines have always existed by Liggs

Faraday said in the show that time is like a street, you can go forward, you can go back but you can't create a new street. You can't change anything, whatever happened, happened. I think Faraday was close, but not exactly spot on. His explaination of why they could actually change things was a bit lacking, but I think there was a deleted scene where he explained it something like how I am going to try and explain it now.

Time is more like a river. Now, you can't create new rivers, but you can redirect the flow, if only temporarily. Desmond did this when he went back and changed things slightly with Penny and Faraday did it when he banged on the hatch door in the past. That was the equivilent of dropping a small rock into a river. It has very little effect on the flow of the river, the river just flows around it and although there's a temporary split in the river where it flows around the rock, very little is actually affected in the long run and the river ends up the same at the end.

The bomb was like dropping a huge boulder into the river. In order for the river of time to flow around it, it had to literally split in two. There are two realities, the bomb didn't go off, and the bomb did go off. Now, this is a pretty huge divergence, we've seen this already in the alternate reality. It will take a while for the river to flow entirely around this obstacle and for both timelines to merge back into the single river.

The important thing to understand here is that the obstacle was ALWAYS there. The universe split in two at that point, that's just how time flows. Once it gets around the obstacle, they will merge into one again.

This, I think, is the explaination for some of the strange things that happened in the first reality that we saw (815 does crash). The timelines seem to mimic each other. This is because time, like a river, WANTS to flow back into a single river. It's trying to course correct to bring the two universes back together, but there's a huge boulder in the way. This is why the universes seem to mimic each other. In reality one, the Marshall was hit on the head by some luggage during the crash, in reality two, Kate smashed his head against the bathroom sink. Both resulted in the same injury.

There are other similarities, Jack and Locke having a conversation about "finding Jack's father". Boone and Locke sharing words. Jack saving Charlie, Charlie almost dying. I'm sure there's more, but that's all I can think of for now.

Now here's the important part. It's not just the new reality mimicing the old one because both realities have existed the entire time. Both realities are mimicing each other!

I might be way off, but Jack offered Locke a consult towards the end of LA X. If Jack fixes Locke in the alt reality. Perhaps the first reality mimics that by giving Locke the use of his legs back after the crash.

There are probably more implications to this but that is all I have thought of so far.