You could also use tar to *append" /etc to the /home archive.
A quick look at man tar, and a bit of reseach into DLT drives, and it seems to me that you could do:
tar -cvf /dev/st0 /home
to begin the archive, then
tar -rvf /dev/st0 /etc
to append /etc to the /home archive. It will take a while longer than michaelk's suggestion because the tape will have to unwind to the end of /home before writing /etc to the archive.