It has to do with how the new cabs and IR blocks are handled by the SHARC processors. They were able to cut the DSP usage by taking advantage of something particular to the SHARC chips, so this limitation is a result of that. You can run two cabs, two 1024 IRs or one of each on a processor path now. I believe you can still use a dual legacy cab block and an IR block on the same processor path, though.