ImageMagick 'montage': How to append images horizontally and to add space between them?

I need to append two images horizontally, that is, on the left and on the right sides, and to add space between them. Here is what I use currently for this:

magick montage input1.png input2.png -geometry +10+0 output.png

But there is a problem: -geometry +10+0 adds space not only on the inner side, but also on the outer side.

magick -size 100x150 xc:red  image1.png
magick -size 100x150 xc:gree image2.png
magick montage input1.png input2.png -geometry +10+0 output.png

(The border is added for clarity.)

Whereas what I want it

I can achieve it using -trim, but -trim is not really convenient for some images, e.g. if the images are book pages, and you don't want to remove their margins. So I wonder whether this can be achieved using -geometry instead. Is it possible?

Use +smush N where N is the width of the gap in pixels:

magick red.png green.png +smush 10 result.png

Use -smush N to stack above/below instead of side-by-side.

Set -background blue before "smushing" to make the gap blue.

Use a negative N to overlap your images.