Adding a second speedlite to the setup is easy. Just turn the speedlite unit with wireless slave capabilities, to the ‘slave’ setting and to the same channel as the master unit, like for the setup with one slave.
In the setup above, both flashes will equally contribute to the total exposure. To achieve the soft light in the portrait of Elena, one 550EX was set pointing to the ceiling, to “bath” the scene with some soft bounce light, while the 580EX in the sofbox provided additional direction light to the face of the girl.

