Why can't you combine .tar.gz files with cat? (alexwlchan.net)

    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