As the Time Machine harddisk (that I am using for my Macbook Air) turned out to be read only for unknown reasons, I decided to buy a new 500 GB unit with 2.5” for 60 € and format it for dual use.
The first (approx 100 GB) partition now holds a bootable Carbon Copy Cloner image while the second partition (with the remaining space) is storing all ongoing Time Machine Backups. I just verified this setup – the hard disk is available for the boot process by pressing the option key immediately after starting the computer with the most recent files residing on the second partition. Looks like a great backup when being on the road and you laptop either does not boot correctly or needs to be replaced.