I haven't tried it with XSI yet, but it works with SI3D. I hope it does with XSI.
Instead of managing the job with one computer, open a batch renderer on each machine and ask them to render the scene. Of course, make sure that they output the frames to the same folder. Because you checked "Skip Rendered Frames", each machine should recognize what each other is doing, preventing them from doing the same frames.
In SI3D, when one process is starting a new frame, it creates a temporary file with the frame number in the render_pictures folder. That file states that this frame is currently rendering, so the other machines search for other-not-rendered-frames.
Hope this helps
Salutations - Cheers
Bernard Lebel