Speedlite 580 EX II Master / Slave HELP
During a test fire the master flash will fire first and the slave will fire second. If you are using groups such as "A:B:C", A will always fire first on a test flash followed by B and then C so you can determine which flash is which, however they will all go off at the proper time when you take a photo.
Try taking a photo without holding the camera to your eye but instead looking at the remote flash while actually taking a photo and verify the flash timing.
Disabling the on camera flash from adding to the exposure is explained in your camera manual, however it will still flash instructions to the remote flash before the actual exposure takes place.