J,
I just did with my gear. You cannot connect both the Camera and the HD on the same FW400 buss. No good. Both the camera and the drives cannot be on the same FW buss.
But, you can connect the camera to the Mac with a FW400 cable to that port and a your FW400 HD with a 400-to-800 conversion cable and use the FW800 port for the drive. That worked fine for me on my MacPro laptop. If you only have a FW800 port, the only other option is to connect the camera with the FW400-to-FW800 adapter cable and get an eSata 34 Card and run eSata drives. (That’s what I use – an eSata Raid.)