Kymmy, you forgot high and low band too!
H/V polarization is switched by changing the voltage on the cable.
H/L band is switched by the presence or absence of a tone on the cable.
This is why you can't normally share LNBs on one cable.
However, what you can do is shift the output of one of the LNBs up the spectrum so that the receiver can see both outputs simultaneously. Trouble is that doubles the bandwidth you need down the cable - hence all the restrictions that Chris mentioned.
You'd have a better chance of this working if you have a descent drop cable - but I suspect it isn't! I've no experience of these but know they're rare - probably for a reason. You might find someone more knowledgeable over here. They're a helpful lot too!
http://www.satellites.co.uk/
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Alternatively, try squeezing the other cable in through a door or window with one of these:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=180305080397