Dual A2DP means that the headset supports 2 x A2DP Bluetooth profiles. This allows a person to pair and connect 2 separate devices that support the A2DP profile to the headset and be able to listen to the audio from both of them. If you have two audio (A2DP) devices connected to your headset then the audio from one device will interrupt the audio from the other device. For example, if you are playing music from the primary mobile phone, then it can be interrupted by playing music from the secondary mobile phone and vice versa.
To use Dual A2DP, pair your primary device by placing the Sena headset into Mobile Phone Pairing mode. Then pair your secondary device by placing your Sena headset into Second Mobile Phone Pairing mode. Now both devices will be using the A2DP Bluetooth profile for stereo audio.