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...I am setting up my flashes tonight in preparation for tomorrows shoot. I have opted to use two Canon 580EXII's on chettah stands. I'm going to use just the flash heads, not the softboxes, but I am using flash head diffusers on each flash. ...My plan is to set these flashes on either side of me, about eight feet away on either side from my position.
I may be too late, but I don't frequent this forum any more; someone PMed me and suggested I weight in.
Ditto what jcolman said about not using small diffusion devices (flip-out lenses, Sto-Fen Omni-Bounce caps, Fong Domes, etc.) outdoors. They're all designed to spread out the light so it can bounce off walls and ceiling to give dispersed bounce fill; if you're outdoors, all that light has nothing to bounce off and will just be wasted going out into space.
What creates soft light is a light source that is large relative to the subject, and bouce caps et al don't make the light enough larger to be worthwhile if it's not bouncing off anything. If it's too windy to use a really big umbrella or softbox, then you're better off just using the straight flash.
As for spreading the lights 16-feet wide, I wouldn't; I'd put the lights close-in on either side of the camera and aimed so each one covers the whole group. (Having two lighst will allow each one to operate at less than full power even in daylight.) Keeping the lights close to the camera laterally will avoid crossed shadows, and also prevent anyone's head from casting a shadow on anyone behind him or her.
The heads should be zoomed to the same setting as the focal length of the lens, or slightly wider, to get full coverage of the group.
Raise the light stands high enough that the light shines down on the group at an angle that will avoid the "deer in the headlights" look, but not so high that the eys are shaded by the eyebrow ridges to the extent that they turn into dark "raccoon eyes."
Also, ditto on the advice to use a step ladder so you're looking slightly down on the group. Don't over-do it though (we have one frequent poster here who likes to get so high that standing figures look like cardboard cutouts lying on the grass); getting some sense of a horizon helps keep the perspective feeling normal, and in your case if the mountains in the background look good you'll not want to be too high for that reason alone.
Good luck, and have fun.