I used to have two outdoor speakers under our patio and two pool speakers connected to zone 2 on my Onkyo 7.2 receiver inside the house. They each had their own separate in wall volume control that impedance matched so I could run two speakers off each channel for 4 speakers total. It worked fine.
Then about two years ago, I wanted to revamp everything. I got a new receiver for inside the house and had another old 7.1 receiver from a friend.
Now I have 5.1 surround in the patio covered area where the TV is (2 front, 2 back, center channel and a subwoofer) for truly awesome movie nights, I added two speakers each to one of two arbor areas and upped the power to all the speakers by giving each their own separate channel. Each pair of speakers has its own in wall volume control.
I did this by using the main receiver as the 5.1 system and running the second receiver as the unpowered zone 2 from the main receiver - the main passes rca signals to the sub receiver which operates in 7 channel stereo. It is all hooked in to an Apple Airport Express that we stream wirelessly to from our phones, ipad, etc.
What all this means is that I have 11 speakers plus a subwoofer that can all be playing the same source at one master volume but you can adjust each zone manually with the volume control down from that volume. Keeps it simple because you don't have to break open the phone to adjust the volume for each zone that way.
Also, the TV area can be playing a movie, etc. and the arbors and pool speakers can be playing music simultaneously at their own independent volume.
We live in the country so it can be played as loud as I want. I LOVE this setup. It is really convenient, didn't cost much (you can get multichannel dedicated amps but they are expensive as heck) since I already had most of the gear and it is perfect for what we need. I also have more channels available if I can figure out where to put more speakers!
The one thing I will say is that if you don't have a subwoofer outside, add one before anything else. It makes the music sound so much better.
I like having multiple speakers because you can have the volume very low but there is music all around you and you can still socialize, etc.