You need to select a stereo input option in the channel sources choice on the track control panel (LHS). If it's possible to pair the two inputs, then this is where you do it.
Alternatively, just don't bother! Simply record two mono tracks and mix them down to a stereo track afterwards. Mind you, if you are recording vocals like this, then there really isn't a lot of point, as your voice is mono anyway, and you only really need a single output from the mic - one side or the other would do. If you are recording ambient sound, then okay, stereo - but you really don't need that for a single voice.