I have a 5DMKII with 580 EXII and 270 EXII (the new one which can be used as a slave).
First I thought I needed only 1 transmitter and 1 flash trigger.
I have put the master 580 EXII on a trigger and the 270 EXII I turned on the slave button.
The 580 EXII flashes wirelessly but not the 270 EXII, seperately they both flash on the trigger.
First I thought there was a sender built in the 580EXII, that sent wiressly a signal that the 270EXII slave would also flash automatically, but I guess i'm wrong and need 2 triggers ?
I use this one: