If hdb is unmounted, and a crash happens on hda, it won't effect hdb at all.
If you have a boot image on hdb, which I assume you do as it is basically a mirror of hda, then you should be able to fine. It should be as much as adding
image=<image path here>
label=Linux on hdb ( or whatever )
to your /etc/lilo.conf
don't forget to run '$/sbin/lilo' afterwards