two sets of "Backups.backupdb"

I'm using TM to backup files on my iMac (OS 10.8.3). I want to regularly backup "Pictures" (and inside that folders my new Aperture3 Library) and "Documents", so I've set up TM preferences to exclude the following:

/Applications

~/Desktop

~/Downloads

~/Dropbox

~/Google Drive

/Library

~/Movies

~/Music

~/Public

/Users/Shared

~/Sites

.../User Guides And Information.localized

In addition to the internal HD I have an external HDD . However... I seem to have two sets of Backup.backupdb folders and I'm not sure what's going where, why and when.

On the HD: HD>Backups.backupdb>My iMac>number sequence, e.g. 2011-08-09-124425 >HD>System>Library>Core Services etc

On the HDD: HD>Backups.backupdb>My iMac>HD>Users>my name>Pictures>Files & folders

My concerns are two-fold: make sure I have a redundant backup of pictures and documents, and have the TM/HD/HDD process set up for best use. Are there some issues in this setup I should modify? Thanks for any help,

Brian