I used to carry only 1 body and wrap the included neck strap around my wrist. That got annoying because every time I needed to do something with both hands, I had to find somewhere to set the camera down. Knowing I really wanted to carry 2 bodies, wrapping the strap obviously wasn't going to work.
After a bit of searching around, and making a similar thread to this one, I found Black Rapid straps and decided to give them a try. I got the Yeti first:
BlackRapid YETI Dual Camera Harness RS2DC-1AL B&H Photo Video
You can remove one of the straps and use it as a single camera strap, but I prefer keeping it as a 2 camera strap at all times. That way, I don't have to readjust every time I use it. Just put it on and go. So, to me, it always remains as a 2 camera strap.
It's comfortable and the shoulder pad is wide and padded. With a pair of K3's, DA*300 and another lens on the other camera, you don't really feel the weight pulling down on you. Even with 2 unequal weights on each side, it stays in place. I never have any issues with it shifting around or trying to slide off. K3 w/grip and heavy lens on one side, and K3 w/o grip and small prime on the other has never been a problem.
For the times where I carry only 1 camera, I use the Black Rapid Cross Shot. The shoulder pad is slimmer and you barely feel it on your shoulder. I've only ever used this strap with smaller lenses, such as the DA15, 21, or M100 for example. My girlfriend uses this same model of strap with a K3 w/grip and 55-300 WR. She loves it.
BlackRapid 63" Cross-Shot Sling Camera Strap RS16SC1O-BL