I second Chaz's response. By meditating before bed you miss out on taking the inner silence experience into daily activity, and the late hour (being tired) could make it less effective. Two a day still trumps one a day.
Over time, as you become accustomed to the practice, you may find it easier to enter DM and then you can 'steal' a quick meditation while riding in a taxi or waiting to board a plane, for instance.