Because zone 2 is independently controlled (both on the 990 and on other processors and receivers), it is not suited to use as a second output for bi-amping. There's an old post by Scott indicating that the unbalanced and balanced pre-amp outputs are available simultaneously, but I'm not sure if both outputs are at the same level (it's common for processors with both output types to have the balanced signals at a different level) and unless half of your space amps were balanced it wouldn't do you any good anyway. The best bet is to go with a reasonably well-made Y adapter.