1) Is it possible to output the audio over a2dp to multiple speakers simultaneously?
2) Is it possible to change the audio output to A2DP speakers directly from iTunes or do I have to change the system preferences every time?
1) I think it's dependand on the speaker set you're using, but I don't think so.
2) No, you can change your audio output in a simpler way:
Try using Audio MIDI Setup, it's in the Utilities folder.
To rename the device, right-click on it once it's in the list and call it something memorable.