I'm not massively au fait with Sky+ recievers, but I imagine they need one cable for the watching tuner and one for the recording tuner - they wouldn't be able to share one. Or, at least, that's how I imagine it'd work. You can't just split the cable at the box end because of the way LNBs work (different frequencies, tuning properties, etc).
You can however get LNBs with more than four outputs - dunno if Sky would fit one or not? With a quick Google, it's fairly easy to find ones that'll give you eight feeds.
