The SPEs on the cell processor are actually pretty good at rendering graphics. In a lot of late gen exclusive PS3 games you can see that the developers utilised them more and more for graphics rendering. So the plan was to have the SPEs on both cell processors do all the graphics.
Would that have even worked? I can't imagine you'd be able to get anything better than PS2 graphics with just an extra CPU.
The SPEs on the cell processor are actually pretty good at rendering graphics. In a lot of late gen exclusive PS3 games you can see that the developers utilised them more and more for graphics rendering. So the plan was to have the SPEs on both cell processors do all the graphics.