You may want to wait a bit longer as the new Bluetooth standard allows multiple devices simultaneously. You can connect multiple device via bluetooth now, but results vary regarding being able to play to multiple speakers at once instead of just having to choose one of the connected devices. Anything you cobble together now will likely disappoint.
I want to give my guests, when we have night sessions, ability to watch movies in my garden but I don't want to disturb the neighbors. They would all use BT headphones and they would listen to the same audio.