SkipD wrote in post #10336944
...that extra light will overexpose the image (unless the second light is positioned so that it only illuminates relatively distant background elements in the scene and NOT the main subject) because the ETTL system knew nothing about the extra light.
This is the key phrase. I've tried what you are asking for, and it does work, if you keep this in mind. In my case, I was taking photos outdoors, in the dark, and used the flash (580 EX II) on the camera, with E-TTL II, directly towards the subject to illuminate that, then another more powerful flash, with sync cable through PC connector, to illuminate the background a bit. The external flash had its own metering (it was a Metz 45), and with some tweaking of compensation for the two, this setup worked well together. But it was not to make any nice modelling light, but just to be able to illuminate action shots in the dark, without all the background being completely like a coal mine.
Now I have more than one 580 EX II, and use them instead for background fill. In such dark situations, that works better, as I can have some assistant running around, aiming them towards the background. In the dark, where there is no competing sunlight, Canon's wireless system has a range that's good enough.