Bluetooth chips are each capable of connecting to a single device at a time for media streaming. If you want to connect to more than one device, you'll need to get a device made to do this. This is called multi-streaming (that keyword alone should get you far in your search) and there are a few vendors who sell dongles which do this.
You can also use multiple BT transmitters. This is only going to be possible from something like a PC; this isn't going to be an option if you're tx from something like a home theater amp or phone or basically anything else. If you go this route you'll need to use software, like pulseaudio, to route each audio channel to the appropriate device.
Either of these type of setup will also allow you to listen to two sets of headphones at once, BTW.
Multi-streaming devices can be a relatively expensive option both in terms of cost and system overhead. You can save a lot if you can get by with a setup that streams once and uses wires between the remote speakers rather than streaming to each speaker individually.
EDIT: In the time since I've answered this question (late '13) a new BT standard has emerged. The new standard is supposed to be capable of multi-streaming. Soon, most BT devices should have this built in.
https://thenextweb.com/plugged/2020/01/07/bluetooths-new-standard-will-support-streaming-to-multiple-devices/