[robespierre]

Dual R10000 @ 250 MHz is slightly faster than single R12000 @ 400 MHz for tasks like rendering—the Octane architecture is optimized around dual CPUs and they can reach their potential throughput. I think the R12000 will feel faster for some other (sequential rather than parallel) tasks.
See Ian Mapleson's benchmarks: http://sgidepot.co.uk/perfcomp_RENDER1_blender.html

The Odyssey V6 is about 70% faster than MXE. See http://sgidepot.co.uk/perfcomp_GFX1_Inve...lebuf.html
Having dual graphics pipes doesn't increase performance, but if you can place different 3D windows on two separate screens, they can use each pipe without contention. There may be some limited use cases for this.
The more common reason that an Octane was provisioned with two pipes is to use the CADduo option for two simultaneous users.