aww ok, I'll have a second look at it, my idea was to have main camera with remote flash, tue secondary camera would fire to have other angle or to have that behind the scenes shot which would not have the correct lighting. I guess my other though then would be to fire everything from a third trigger.....hummm just thinking out loud.
You can do that. But only with one on camera flash, not remote flashes and groups.
Set your main camera with on camera flash to Linked Shooting Master. You have to set P.Fn-07 to 1. Set your second camera with the YN-E3-RT on Linked Shooting to Slave. You are good. When you press your main camera shutter the flash will fire and the remote camera will be triggered. The flash will not fire in sequence with the remote cameras shutter, only the main cameras shutter, they are not perfectly syncronised.
To get into Linked Shooting press and hold the lightening bolt button.
When you setup the cameras like that you get the flash menu and the REL button, with that you can trigger the remote by itself by just pressing the REL button on the main cameras 600. Of course f you press the shutter button both cameras are triggered.
To get it to work with remote flashes and groups you'd need a separate trigger for the remote camera, a $20 set of Yongnuo RF-603's would work fine.