The simplest way is to export a map with proj 90 and then export the extended with the pro 90 map as an image or vice versa
layout layout
map-header 0 0 off
endlayout
layout image
map-image 0 100 sw elevation90.pdf
map-header 0 0 nw
endlayout
export map proj 90 -o elevation90.pdf -layout layout
export map proj extended -o elevations.pdf layout image
How to combine projection and extended elevation
into single picture?