tar -c x | head -qc $((18*-512)) | gzip > x.tar.gz tar -c y | head -qc $((18*-512)) | gzip > y.tar.gz cat {x,y}.tar.gz | gzip -d | tar --list seems eas

tar -c x | head -qc $((18*-512)) | gzip > x.tar.gz tar -c y | head -qc $((18*-512)) | gzip > y.tar.gz cat {x,y}.tar.gz | gzip -d | tar --list seems easier to just not have the EOF marker, if you need to append stuff later