you should be able to do that, make sure you name both images different names so they do not conflict, as well as so you know which is which.
just also remember if you do a periodic back up image that has updates and whatnot and saving it to the same HD that any errors on your system and such will be in that periodic Image. and also if you get a Virus and not sure of it once you plug that external in or copy a new periodic Image to it that Image now has the Virus and so does your External HD. has a chance to Contaminate both, not a guarantee though.
the point of the Image is so that you can have an emergency restore back to defaults. best option would be disconnect network cable, install windows, install antivirus, use a second computer and download anti virus updates if you can. update the anti virus with those downloads, reconnect to network update windows and antivirus/firefwall. then Image the machine. then you will have a system that is somewhat defended. have that Image on a DVD or a 32gig Thumb Drive.
then on that external you can have the same Image and the periodic Back up, that way in even your HD goes bad or a contamination you have the original backed up.
but in short yes you can do both Images on one Drive, but its not highly Recommended
hope that was of a little help at least