Which connected to the phone and the charge. Both are connected is not necessary (they are connected to each other do), and if you connect both the main one that first got out of the case.
They have a working principle. When you goes to the earpiece from the case he first tries to connect to the second. If it does not find, it is connected to a device which has been connected.
Although if you connect the two, do not we all somehow works steadily.
Therefore it is better to connect one of some sort.
If you have already connected to a phone, then get two, so that they are connected, remove the phone and remove the back case. Then get the one you want to make the main, to connect to the phone and then get a second, which is on the machine to connect to the first.
You can also connect it to two devices. To remove them in this case, the second control (which is not connected to the phone) to connect it to the second device and after the first control, which is on the machine to connect to the second earpiece.
Thus, they will connect to different devices, depending on what you got the first earpiece (second take a few seconds to fully engage the first managed).
But it is advisable to disable the Bluetooth devices, where it is not needed as sometimes the headphones can be connected each with its own device, and more with each other.
Then start to each other to interrupt the audio stream. That is, both the earphone will play the sound from a single device, but if the second device, including songs, this flow would kill first and both the headset will play the sound from the second device.