How Bluetooth Works
iOS: Supported Bluetooth profiles
iOS: How to troubleshoot Bluetooth connections
You can connect multiple bluetooth devices, like a keyboard and headphones. However, not two that use audio. That's because the audio channel over A2DP can only play to one device at a time. Its a limitation of the audio feed via bluetooth - there is no split or simultaneous feed to multiple devices. A2DP is a uni-directional 2 channel (ie. stereo) audio feed only.