I have a K100DS and K20D. I leave the 18-250 on the K100 all the time, so it's always ready to take a photo. The K20 has whichever lens I used last, which may or may not suit the scene.
I have a very lightweight Travor grip on the K100, so that it balances beautifully with the chunky 18-250 and Sigma flash. I never use the Sigma flash on the K20, I only use it in wireless mode, because the K20D/BG-2/Sigma flash combo is too heavy and uncomfortable (the front e-dial gets in the way of my index finger). At family events I like to use the K100 with 18-250 and flash mounted and the K20 with a fast prime.
I intend to get a super telephoto lens eventually for wildlife shooting, and I will carry the K100DS to avoid lens changes.
It's a lot of fun to have two bodies, and comforting to know I have a backup, just in case. Sometimes I loan the K100 to my wife so she has a something to do when she accompanies me on a photo jaunt. What I don't like is that she has better ideas on composition.