I just had our soapstone installed and this is how they did ours. I was fine with it and it looks great. The seam is invisible and you have to really look to see if there is a break in the patterning.
I think their ask is quite reasonable.
caspian

Perhaps it would help if you took photos of the proposed layout, cut up the photos and pasted the pieces together at the seam. Maybe it will be fine, but maybe you won't like it. Helpful to find out ahead of time.

If it's real stone it will have seams. No product is perfect. If you don't like the layout with one slab, find the original quote and refer to that when negotiating alternatives. You are probably right about the savings [for them] when using fewer slabs.

It sounds like you're paying by the square foot, since you say that using only one slab won't save you any money. In essence, you'd be doing them a favor. If that's the case, perhaps you can ask for a reduction in price. They can justify the discount to you, because they can make more money with that whole slab you saved them. Without benefit of a discount, I'd press for using the two slabs since that's how they priced it from the beginning. The cynic might accuse them of bait and switch.

I had a similar situation but it was me who was trying to save money. My fabricator said he could fit everything on two slabs but it would be tight with no room for custom templating (there was only one way to make it fit.) We saved quite a bit of money not having to purchase an additional slab but we ended up with one seam that doesn't quite match. The tones of the granite are just different enough that the seam really stands out. Fortunately the seam is at the cooktop so it is short. The veins may not matter but you may want to check if there is any color variation, ever so slight, from the two different pieces.

Mine are marble but similar kind of dilemma - I had a choice of two slabs. The slab I preferred would have required a corner seam. The other slab would not. At first I planned to take the slab I preferred (it had slightly more markings) but slept on it over night and in the morning changed my mind. Am really glad I did too, as I love the marble and very happy not to have the seam. My opinion is: Try to avoid the seam if you have a choice to do so.

I'd try to avoid the seam if possible. Our perimeter was small enough, that there was no seam at all. I've thought about how glad I am about it. The whole time we were looking at soapstone, I assumed we would have one. Someone posted their soapstone island a few months ago, with a seam down the middle, and the veins totally didn't match up. If you have to have the seam, make sure there's minimal or no veining. All the best to you.