When working with a two-flash set up on the go, you can easily change the power ratios between your main light and the fill light (without having to fiddle with moving light stands around). For my rig, I use the Canon Speedlite Transmitter ST-E2 with a pair of Canon 580EX II flashes.
To set up your rig, make sure the flashes are in slave mode and the transmitter is secured in the hotshoe and turned on. Set the 580 EX II flash on your left to "Group A," and the flash on your right to "Group B." Then all you have to do is press the Ratio button on the back of the transmitter, and choose the lighting scheme using the two navigation buttons to the right of the Ratio button.
For portraits, I usually start with a 4:1 ratio with this rig. But if I don't like the effect, I can easily adjust. I have a total of 13 ratio settings to choose from! This set up works great.