It is always handy to have a backup camera. I do take both cameras with me when I travel. Last February I photographed Yukon Quest the international dog sled race and was able to capture some amazing images. I used both cameras and paired them with the FA 24-70 and my Sigma 70-200 lens. I frequently swapped lenses depending on the job at hand. Usually the longer lens was on the K3 and the shorter on the K1. That way I was able to capture a dog team coming towards me first with the 70-200, fill the buffer and then when the team was close to me switch to the K1 with the 24-70, fill the buffer and then as the team moved away, switch back to the K3 to complete the exercise. Worked perfectly. Here is a link to a sampling of a days photos
Yukon Quest Day 4 | Flickr