Crazy thought here…I’ve never tried this and it goes against conventional final Cut logic but…
What if you use a USB drive for your render drive? We all know you should not use USB drives for editing, but during the render process, the computer is just dumping off little pieces of the rendered timeline to the disk. No real time playback involved at all. The Firewire and USB busses should be different animals and shouldn’t interfere with each other (too much). After the render is finished, copy it back onto your Firewire drive for proper playback. You should be able to preview the final render from the USB drive to verify it’s there, but put it back on the Firewire drive for use after that.
This way all source files are coming from one drive and all rendered files are going to a different drive. Renders should only be limited by computer speed and drive through-put.
Might be worth a try?
If you do try it, let us all know if it made any difference in the total render time. If it works, it might be a good work around for people cutting on laptops with a single Firewire port.
E=MC2+/-2db