I wouldnt cut my seat in 2, unless I had a spare one.

Wel,, then its your baby,, Good luck,, I dont think the seat pan is built right for cutting it in 2 &  it working out  to recover nicely,, I guess youve looked at it, decided where to cut & how youll recover it? Is there sufficient strength across the back edge after its cut? The point Im trying to make is, when a piece of plastic is formed, its designed to handle loads, change the shape by cutting it, then the edge may not have the right reinforcement to handle to job..

I thought about buying an older two piece seat. Not sure how they mount though, and I'm not sure what the stock foam looks like - or what its quality is compared to the foam I currently have to work with.

I took some pics to show what I've done so far. Once they are transferred from my phone, I'll upload them.

In my opinion, here is the flaw in the original seat. First, the back of the seat is flat, even convex a little. There is a flat ridge where it meets the rear seat. This design creates a hot spot right in the back of your rear, and right on your tailbone. Second, the flat part of the seat is also a bit convex, so it doesn't cradle you at all, it's more of a hump you're sitting on.

Basically, I ended up taking this rough scowering pad that I had in my tool area, and I started shaping the seat. I also used a round sanding disk in my drill, but controlling it was difficult at best ( although for some edges, it works quickly ) The hand scouring pad gives precise control over where you want to shape the foam. In the back I removed the edge where it meets the rear, and I've also curved it in a concave fashion. I took out a little foam where the tailbone area lands. I also started reshaping the flat part of the seat to make it just a tad bit concave, so it will cradle a little bit. So far in my testing, it feels way more comfy. I'm going to lift up the foam from underneath. I'm using this black dense rubber foam that was part of an exercise mat. I may also cut out a place for this same foam on top, because it is way more comfortable. It's has a more gel like consistency.

I've cut the rear foam off, but I did it carefully and I may reattach it. Not sure yet if I want to make it a two piece, although I think it could work.

Pics to come...

I trimmed some of the foam and recovered it. The shape was alot more comfortable but i took off too much foam. Decided to go with a sportster seat which was better than the stock seat but not too much better. Then I got a corbin seat and I can stop looking. I can ride all day on that thing. Here's pics of the trimmed stock seat.
AL

Update. I put the seat back together temporarily, as a one piece, so I could take a test ride.

I put some of the black foam underneath the seat foam, and I put one piece on top. I'd say I lifted myself up at least an inch, and I gave myself about an extra inch or in the back. I'd have to say the first test ride was a complete success. Much more cushy and the curve in the back totally relieves that pressure on my rear and tailbone. ( I can't really know without driving those 120 miles again, but I can get a sense of the comfort level )

I'm not sure yet if I'll stay one piece for go for two. Also not sure if I'll inset the top black foam into the original foam. That would lower me back down some. I'm not sure which would be better for my back - sitting up higher or lower.

I'll post more pics as I take it back apart. That'll show what's going on underneath the stock foam.

I am glad you are posting these pictures because I have been thinking about modifying my seat.  I want a different form than you are making but the technique of reshaping a seat is the same.  Your statement about it absorbing more impact than the stock seat makes this mod especially interesting.

A real Game Changer for me was when I strapped a bag on the back seat that gave me a place to lean my back side against.