The easiest way is to actually have a separate data partition mounted from both. About as easy at least when setting up the partitions would be to have one /home partition shared between both. This way you can share more than just your HOWTOs but data and settings as well. Now that you're all set up with them this could turn out to be rather tricky, though.
Another solution I just tested and works fine is to mount the partition somewhere like /mnt/hde. /mnt/hde then becomes your "root" for that partition so for example your howtos might be in /mnt/hde/home/user/howtos instead of /home/user/howtos. Now that it's mounted on your working OS you can link to it wherever you want.
ln -s /mnt/hde/home/user/howtos /home/user/howtos
If you make the mount permanent (I.E. in /etc/fstab) then your link should never break. It won't work unless that partition is mounted. You also want to look into mounting that partition as user or you will probably have to be root to change anything in that folder.