What you seems you want is a "union" mount, which can't be done in Ubuntu (at least not with default tools; follow the above link you are adventurous). In general there is no official union-mount solution in Linux; there are out-of-tree filesystems like AUFS and work-more-or-less-in-progress on UnionMount. Valerie Aurora has a (slightly outdate) page about it.
What you can easily do is mounting the disk at, for example,
/mnt/disk1 and then symlink the directories where you want them:
ln -s /mnt/disk1/home /home
ln -s /mnt/disk1/shared /shared
(again, you should not have an existing
/shared folder, confusion and/or errors would appear otherwise).
Another way to do these kind of things is a "bind mount". It's very similar to symlinking directories, but can cross "chroot" boundaries. You can find more info in this page and in this other one.