Unless you really need to share sources, (and even if you do) it might be easier to just get a 2nd receiver for outside. You can always use the receiver as an amplifier (just patch your output into one of the receivers inputs) but you'll have the benefit that you can use it independantly of the one inside without a lot of hassle. IE I just want to be able to listen to the radio outside, it's a lot easier for me to wire up a 2nd cheap receiver, then run cables all through the house, get a seperate remote IR sensor etc.
Another thing that pissed me off, even though I have a $1500 yamaha receiver, the "2nd zone" only works off analog, not digital, inputs. So I have to double my wiring (a lot more than that actually, since I'm only using thin optical cables) to be able to support the feature...