Unless you truly need to treat two directories as the same one (unlikely) there's no need to use symbolic links or even LVM for that matter to accomplish your goals as the `nix filesystem is designed in such a way that any storage device partition can be mounted to any place on the tree. For instance you can mount /dev/sdb1 directly to /home, /boot, /home/Videos or anywhere where you are running out of space.
You will need to move the existing content to the new storage location prior to mounting but other than that it's a snap. I recommend using live media to boot from to perform the move.
Here's a general overview of Linux filesystem (Unix is the same or at least very similar).
Here's some more useful info from the official documentation. It details the steps necessary to move \home to another device or partition. The same approach can be taken for any folder on the tree. Usually what's required is to identify the directory that's hogging the majority of your space , copy it's contents to another more roomy device, and mount that device on the previous location.
If anything about this answer is unclear, please drop me a comment and I'll clarify to the best of my ability.