Support for multiple AirPlay speakers?

Can you be more specific about what you are trying to do?

Using AirPlay for audio output is generally controlled by the user. There is nor currently a way to specify multiple audio output destinations in the IOS slide up control.

When playing from iTunes on a Mac you are able to specify multiple outputs for audio, not video, so it is doable for audio contents, although I do not know if there is a high level API available.

The new Apple TV has the ability to select audio output through available airplay speakers. I'm wondering if it's possible to output to multiple using the ATV without the need for a Mac.

I had not noticed that. The Apple TV does not require a Mac, so if you can output to multiple AirPlay speakers from the Apple TV I think you answered your own question.

I'd expect output to be one airplay target at a time.

if you need simultaneous/multiple outputs, feel free to use the bug reporter below to make your request heard.