I would keep both cameras
I would keep the 20D. Using the 20D with the 10-20mm and the 30D with the 17-70mm, you would have a focal range of from 10-70mm. The use of a second camera (especially on an OPTECH Reporter strap set-up) really adds versatility to your shooting.
http://www.amazon.com …trap-Optech/dp/B000819P98
If you add a tele zoom to your lens battery, you could have an unbroken focal range of 17 to 200 or 300mm (depending on which tele lens you choose). I use a 17-55mm f/2.8 IS lens in conjunction with a 70-200mm f/4L IS lens on my 30D and 40D cameras and I am in love with that combination.
Using a second camera also adds an insurance policy. I recently tripped climbing a slippery slope on the first day of a ten-day Kenai Peninsula, Alaska trip. My 40D which was on a monopod, landed between my jaw and a rock. I cut my lip and broke the camera (which has since been rapaired back to like-new status).
If I did not have my second camera with me (a 30D) I would have ended up with virtually no photos of a very interesting and picturesque ten days.
Murphy's Law states, "The likelyhood of a piece of gear failing is in a direct relationship to the importance of the photography and in an indirect relationahip to the availability of backup equipment!"