Can an iPhone stream to multiple Airplay devices at the same time? 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.