You can use a headset as a microphone instead of the mobile phone, but to do so, you should use the device's 'Listen & Play' mode. In this mode, the mobile phone captures the audio while the headset provides the translation. However, to use the second headset as a microphone, you may have to configure the device so that it correctly recognises the audio input. Be sure to consult the manufacturer's instructions for further details on the optimal configuration.