Stitching- meet in the middle

[stacsk]

I have both a grayscale and a colored version of the same image. What I wanna do is stitch the grayscale version with the color version, and have an image with the right side gray, left side colored. I can achieve this by using the split tool on both images, and use the stitch tool to combine the halves. But I'm trying to find out if it's possible to do this in one step, by using the stitch tool only.

Assuming it's possible, my problem is I don't know how the "spacing" value works. When I set it to 0, both images are apart, naturally. When I set it to a positive value there's empty space between them. So I assume setting the spacing to a negative value would push the left image to the right, and the right image to the left, making them meet in the middle at some point. But I only see the right image superseding the left image, despite "center" alignment being chosen. And the alignment options (left, right, center) seem to do the same thing, I don't see anything changing when I switch.

Alignment is still "center" here, and changing it doesn't affect anything.

[T8RIN]

You can use compare tool, here you can export result where two images is stitched in middle