By way of an update on my original question, I've run a few test now and discovered a few things...which may help others trying to do the same thing;
1. The Remote App connects to running versions of iTunes software and not directly to the iTunes library, which scuppers my idea of connecting direct to an iTunes library residing on a dumb network drive (without an OS & iTunes software running) such as my Time Capsule.
2. To play two different streams of music simultaneously and then point one stream at one AirPort Express or airplay speaker, and the other stream at a differnet AirPort Express or airplay speaker, I need to have two separate instances on iTunes running at the same time. Given that I can't run two instances of iTunes on the same Mac Mini, I would have to have two Mac Minis...which gets expensive.
3. The good news is that Airplay seems to allow two computers, each running iTunes at the same time, to share the network and point differnet music streams at different networked speakers.
I now have another question: can I take a Mac Mini, partition the drive, install Mac OS on both partitions, open iTunes on both partitions and then use the Apple Remote to switch between the two? If I could, problem solved....I think.
You don't need two computers, you just need two devices that can send to airplay speakers. The iPhone and iPad can both do that, independently of iTunes - just use the "Music" app, not the "Remote" app.
It does mean that you'll need to either sync your iphone or ipad to have a copy of the music stored directly on the device, or subscribe to iTunes Match ($25 per iTunes account, and you get access to all your music without needing to sync, provided you have network access - Amazon and Google have similar services). Or use Home Sharing iOS: Setting up Home Sharing on your device
Also, if you double-tap the "home" button on the iphone or ipad, then swipe the bottom of row of icons from left to right until you get to the volume controls - you can send any audio that's currently playing to your airplay speakers, even from Pandora or last.fm or Angry Birds). There are probably some apps that will stream directly from your dumb network drive - I don't use them so I have no recommendations.
I suggest trying Home Sharing, if that doesn't suit your needs go for one of the cloud services.
Thanks Joseph. The only problem with running everything off the iOS devices is storage, or lack of it. I've around 150GB and I've it will be double that by the time I'm finished putting all my old vinyl in. And the latter, that an the lack of any data associated with a huge chunk of my wife's music, make the Match option less than ideal. Simply put, a great deal of our music won't be found by Match. A friend of our found this with the same, vinyl recordings and club music from dubious sources and ripped from rips, from rips, etc.
I spend the weekend at a friend's house who had the Sonos system and it allowed similtaneous music streams to differnet zones all over the house. I was hoping to try and replicate this ease of use with my Apple kit.
Use Home Sharing.
The music stays in iTunes on your Mac. Then you use the iPhone or iPad's Music app to connect to the shared library, the music will stream to the iPhone or iPad (no permanent copy left behind). If you turn on airplay too, you're using your iPhone to pull music off the Mac and send it directly to the streaming speakers. It's just passing through the phone, without stopping.
Try it. Don't overthink it.
Joseph, unfortunately that's where we are now. The original point was to avoid having our laptops open and on the whole time. I wanted to replicate the sonos system which has a little network storage device sitting out of the way, and you just pick up a sonos mini tablet or iPad/iPhone, and tell it what music you want in what room - really simple.
The other problem with home share and using the iOS Music App is that, unlike the remote app, it wont send to multiple airport expresses or airport speakers.
Any thoughts on a partitioned Mac mini?
You can use Home Sharing on the Mac Mini.
Partitioning makes no sense.
There's one mistake in your reasoning - you said "Given that I can't run two instances of iTunes on the same Mac Mini..." - but that's incorrect. You can set up two user accounts on the Mac Mini (using the "Users & Groups" system preference), turn on "fast user switching" and have iTunes running in both accounts simultaneously.
Here's how it would work.
1. Log in to the Mac Mini, using the "Mr Badger" account, start up iTunes and turn on Home Sharing (signing in with the "Mr Badger" AppleID)
2. Use fast user switching to log in to the "Mrs Badger" account, start up iTunes and turn on Home Sharing (signing in with the "Mrs Badger" AppleID)
Once that's done, you can turn on Home Sharing on the iPad and the iPhone - the AppleID you use will determine which music library you see, you then send the music to the location of your choice. You can also play a song on iTunes on the mini at the same time, and use airplay on that too.
I just tried it, and it works. I had three songs playing from two music libraries, both located on the same iMac, all at the same time.
I got to the User switching as I was droppoing off to sleep last night. I don't know what I was thinking with the partitioning - I've got five iMac at the office all running multiple user accounts, doh!
One question, If the two instances of iTunes of the two User accounts are on the same Apple ID, can the Apple Remote App see both libraries on the Apps home screen? It would be great to control all music from a single device.
Thanks for your help.