Forget to ask. Which camera? If it has pop up flash you might be able to trigger them optically. Set both flahses as slaves and choose optical transmission (the lightnig bolt) on your 600.
If you want to keep the 600 on your camera set it to optical transmission and it will fire your 430 off camera.
Just another thought. If you don't have a pop up flash you get a long off camera cord you can connect it to the 600. Set it as a master, choose optical transmision and set your 430 as a slave. Your 600 will then trigger the 430. This avoids getting triggers if you don't want them. You can't trigger your 600 off camera using the RT setting on your flash. No cameras have RT capability on thier own. Only optical if they have a pop up flash.
To fully use the 600EX-RT's radio capabilty you need at least one RT flash or transmitter like the ST-E3-RT on your camera and another RT capable flash off camera. Only problem is if you are radio mode you cannot fire your 430. It only has an optical receiver. The 600 can transmit/recieve either in optical or radio, not combined. The ST-E3 only works in radio only so gain it would not fire your 430.
http://ocfgear.com
Good luck
