You remember correctly: the 603 was never intended for multiprocessing. It was amazing it worked as well as it did on the BeBox, but AFAIK it was the only 603-based computer ever to try. The G3, the 603's more-or-less direct follow-on, doesn't support it either. From the BeBox FAQ: "We use the Motorola 105 'Eagle' support chip, which allows for either one processor and an L2 cache or two processors without L2 caches. So, our two-processor PowerPC BeBox has no provision for an L2 cache."