I really love AirPlay so I thought about buying a second AirPort Express for another amplifier in the apartment. But will my iPhone5/iPad2 be able to stream to both AirPlay devices simultaneously?
No, you want be able to stream music to two airport express units simultaneously from an iOS device such as an iPad or iPhone.
However, you can stream simultaneously from iTunes running on a Mac or PC.
For example, you can't have speakers playing in the kitchen and speakers playing in the dining room at the same time unless they are connected to the same airport express. If you have them connected to two airport expresses, you can choose kitchen or dining room but not both.
You can play to both if you stream from a Mac or PC using itunes. However, there are apps that will let you control the Mac or PC from your iOS device but you can't do it natively.
to stream itunes to single/multiple/all airplay devices, you will need multiple airplay devices, itunes on a pc or mac (ensure this itunes has your favorite library), then from any ios device, download and install Apple Remote App. remote app allows you to "remotely" control the Itunes library on the pc/mac, allowing you to multi select your airplay devices right from your phone. looks, feels, is exactly like Ipod/Music app on your ios device.
i have been using it for years, but still waiting for a NAS maker to include this 'Itunes SERVER Airplay" functionality right in their firmware. Apple should build it into their nas product (unless they have lately) 6x airport expresses - wired Ethernet, audio out to 6 sources on a multi-zone amp, controlling 22 speakers (grouped in 6 zones) better than most 20k solutions. and all zones are perfectly synchronized. works like a charm.
You can tell the copy of iTunes that you are using to stream to both AX's so the same goes for the iPhone. I have two ATV2's and can use the remote to make music play on both plus my Mac Pro. But.... I don't know the technical reason for this but the music plays on each ATV2 slightly out of sync, it sounds odd. So if I play the same track in the living room and kitchen at the same time there is a strange effect and it doesn't sound right. Of course your house may be a lot bigger than mine and you may not notice this.
With the iOS app Airplay Multiroom Player by WHAALE it is possible to send music to multiple speakers at the same time!
This is a major problem with multiple airplay devices on iOS 7.0.2 we used to be able to stream to multiple devices now I can't:(