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A simple but effective set-up for PJ style candid flash photography with one or two flashes is to keep one mounted on a bracket 12-18" over the lens.
The bracket is a critical element. The most unflattering things about single flash photography are flat lighting when its close to the camera axis and poorly placed dark unfilled shadows if it is moved slightly to the side. The first thing the bracket does is hide most of the shadows from the POV of the camera. In a full face shot with a bracket the pattern is "butterfly" with the nose shadow falling directly below it (helping to hide the nostrils if in view), and defining the shape of the chin and front of the face with the shadow it does cast which are seen. Direction of the light is what creates modeling. Raising the flash creates a natural looking downward angle for the light that models a human face in a flattering way by getting light under the brow and into the eyes, creating a natural looking highlight / shadow pattern with highlights on the raise parts and shadows on the lower ones; the same pattern we see in natural light that comes from overhead.
When you use more than one flash the one on camera becomes the fill source for the off-axis light. In terms of logisitics it is very simple because there is only one off-camera stand to wrangle. That can be make even easier by using a rolling stand. It is the overlapping of a key light over an even foundation of fill that allows the flash lit foreground to exactly match the range of the sensor to record detail from the darkest shadows on a black suit to the delicate details of a wedding gown — at the same time.
In many situations like action shots, or whenever you can't precisely control the alignment of the faces to the key light, the best two-flash strategy is to wheel the off camera flash around to the back out of frame and use it as back-rim light to add an illusion of 3D. Again that's where the bracket is invaluable because it will ensure the frontal lighting on the subject comes from a flattering downward direction without any distracting shadows hanging off the nose or hiding the detail on the front.
The backlight scenario is similar to shooting outdoors with flash where the sun is providing the rim light and the front of the face is in shade which is 3 stops darker because the light comes from the indirect skylight. When you add flash to the face >>> raised on a bracket <<< it creates highlights on the face, over the foundational sky fill. It's really the same as using flash on a bracket indoors, except the ambient light from the sky providing the fill is brighter and makes the shadows appear lighter than indoors where the fill is created by spill bounced off the ceiling.
With that outdoor scenario the shadows remain quite dark and there's not much you can do about it because the flash isn't reaching them and must be exposed for the highlights. For full control over the lighting ratio on a face outdoors you need the same strategy as indoors: a foundation of even fill with overlapping key light. The flash on the bracket reverts to the role of fill (augmenting and lightening the sky filled shadows more) and you add the second flash off axis to create the highlight pattern on the face over the sky+flash fill. Again its exactly the same concept as lighting a face with key and fill indoors, with an assist from the brighter ambient fill from the sky.
That's the simple but effective strategy requiring a bracket, one stand and two flashes. In terms of equipment to implement it? Just about anything can be used. I first used it in the 70's with huge single power Graflex flashes, then for 30+ years with pairs of Vivitar flashes in manual mode with a simple optical slave. When I switched to Canon DSLR I continued to use the Vivitar flashes for a year, but then switched to a pair Canon 580ex flashes. The Canon wireless systems has pros and cons.
The positive attributes are convenience of fingertip control of three groups of flashes in manual mode, and integration with the camera's metering for control of three groups in ETTL mode. M mode provides consistency when shot-to-shot consistency is vital (The Stobist "ethic"), but ETTL is also an option for situations where there is no time to meter and adjust lighting ratios.
The weakness of the Canon system is the need to work around the constraints of the coded optical signaling, and when ETTL is in play the quirks that come from metering flash based on pre-flashes. Because I'd used optical triggers on my Vivitars for many years I was familar with the limitations of optical triggering and found it easy to work around them. First I don't expect it to trigger my slave from 100' away, which is OK because I seldom use it more than 40' away, which is the effective signaling range of a 580ex master pre-flash. Secondly common sense dictates that the sensor on the slave be oriented towards the Master's light and no blocked. I work around that problem by using diffusers I created which sit on top of a vertical flash head making it easy to align the sensor towards the master and the light in any other direction. With the lighting strategies I use I don't have a problem keeping the slave within the footprint of light from the master flash in the bracket. Occasionally I'll use an umbrella on my off camera light but for most applications I find that the small diffusers, combined with the strategy of overlapping the off axis light over a foundation of neutral fill, provides acceptable results.
Would this strategy meet your needs? I have no way of knowing, but its one of simpler one's logistically you might want try first. All you need is a bracket, OC-E3 cord, a 580ex / 580exII Master, and a compatible slave mounted on a stand with an umbrella bracket. Since the off camera light winds up needing as much or more output than the fill in most situations I suggest a second 580ex / 580exII. I prefer the original over the mkII. I you find that doesn't meet your needs you can add radio triggers to extend the range and allow you to bury the slave flash inside modifiers.
See: http://super.nova.org/DPR/Canon/MultiCanon and http://super.nova.org/DPR/DIY01/