Some apps, like Remote.app, can broadcast to multiple Airplay speakers at once. Others seem limited to one speaker at a time. Is there a way to enable apps that use Airplay to have the option to broadcast to multiple speakers at once, like Remote app can?
This isn't directly a feature of Remote.app but of iTunes on the desktop, Remote.app just gives an UI for this. At this time, only iTunes running on Mac/Windows or other desktop apps like Airfoil are able to distribute to multiple speakers at once.