Q: How can I make more than one backup at the same time

I suggest you do not use Time Machine for the second backup. Instead, I suggest cloning your drive to a different backup drive using different backup software.

Your secon drive should be the same size as your computer's drive. Start by using Disk Utility's Restore option to clone your computer's drive to the new backup drive:

Clone Lion/Mountain Lion using Restore Option of Disk Utility

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

     1. Select Disk Utility from the main menu then press the Continue

         button.

     2. Select the destination volume from the left side list.

     3. Click on the Restore tab in the DU main window.

     4. Select the destination volume from the left side list and drag it

         to the Destination entry field.

     5. Select the source volume from the left side list and drag it to

         the Source entry field.

     6. Double-check you got it right, then click on the Restore button.

Destination means the external backup drive. Source means the internal startup drive.

After this initial clone you can use other backup software suggested below to make incremental updates to the backup. These incremental updates can be done on a regular schedule.

Suggested Backup Software

  1. Carbon Copy Cloner

  2. Get Backup

  3. Deja Vu

  4. SuperDuper!

  5. Synk Pro

  6. Tri-Backup

Others may be found at VersionTracker or MacUpdate.

Visit The XLab FAQs and read the FAQ on backup and restore.  Also read How to Back Up and Restore Your Files.

The two backups need not be done on the same schedule. Time Machine backsup hourly. The other backup could be done on a daily or even weekly schedule.