The problem that occurs to me when doing this inside a surround receiver or processor is signal processing. If you have one HDMI input going to one output and another HDMI input going to a second output, which one is receiving the audio (and often video) processing that is one of the surround processor's principle reasons for existence? Which one is feeding audio to the main zone? Basically, whichever one is going to the second output is going to have to be a pure pass-through of the HDMI signal - the expense required to afford the second output any sort of real audio processing capacity is going to get ugly fast, which is hard to justify for a feature that very few users will ever need.