Originally posted by Pentaxor I agree with the K20D as a back-up. but I would suggest that you should wait for 3 more months for Pentaxs next dlsr.
If you want a backup able to handle the same environmental shooting conditions as the K-7, then yes, I'd go with the K20D. If you want a second body that will expand your capabilities in terms of low-light / high ISO shooting, but also be able to fill in for the K7 in many conditions so long as it's not wet or below freezing, then you might be better served by the K-x.
This is one of these situations where you need to be asking yourself what you want the second body to achieve, and you'll likely find the answer is thus prescribed for you.