Based on answer from Gilles and how to find PDF page count I wrote

#!/bin/bash

pdforiginal=$1
pdfood=$pdforiginal.odd.pdf
pdfeven=$pdforiginal.even.pdf
pdfout=output_$1
margin=${2:-0}
scale=${3:-1}

pages=$(pdftk $pdforiginal dump_data | grep NumberOfPages | awk '{print $2}')

pagesize=$(pdfinfo $pdforiginal | grep "Page size" | awk '{print $5}')
margin=$(echo $pagesize/2-$margin | bc -l)

pdfjam -o $pdfood --trim "0cm 0cm ${margin}pt 0cm" --scale $scale $pdforiginal
pdfjam -o $pdfeven --trim "${margin}pt 0cm 0cm 0cm" --scale $scale  $pdforiginal

pdftk O=$pdfood E=$pdfeven cat $(i=1; while [ $i -le $pages ]; do echo O$i E$i; i=$(($i+1)); done) output $pdfout

rm $pdfood $pdfeven

So I can run

./split.sh my.pdf 50 1.2

where 50 for adjust margin and 1.2 for scale.

Based on answer from Gilles and how to find PDF page count I wrote

#!/bin/bash

pdforiginal=$1
pdfood=$pdforiginal.odd.pdf
pdfeven=$pdforiginal.even.pdf
pdfout=output_$1
margin=${2:-0}
scale=${3:-1}

pages=$(pdftk $pdforiginal dump_data | grep NumberOfPages | awk '{print $2}')

pagesize=$(pdfinfo $pdforiginal | grep "Page size" | awk '{print $5}')
margin=$(echo $pagesize/2-$margin | bc -l)

pdfjam -o $pdfood --trim "0cm 0cm ${margin}pt 0cm" --scale $scale $pdforiginal
pdfjam -o $pdfeven --trim "${margin}pt 0cm 0cm 0cm" --scale $scale  $pdforiginal

pdftk O=$pdfood E=$pdfeven cat $(i=1; while [ $i -le $pages ]; do echo O$i E$i; i=$(($i+1)); done) output $pdfout

rm $pdfood $pdfeven

So I can run

./split.sh my.pdf 50 1.2

where 50 for adjust margin and 1.2 for scale.