I've not heard of this one! Don't know about the law, but the MOT manual seems to accept a twin light setup, and doesn't say anything about age of the bike:
On twin headlamp systems only one or both headlamp may be illuminated for either dipped or main beam. Each headlamp that can be illuminated on dipped beam must meet the requirement for dipped beam headlamps. Twin headlamps must be mounted either one above the other or symmetrically without regard to any side car and not more than 200mm apart measured from the edge of the reflecting surface. Full text is on MOT website:
http://www.motuk.co.uk/mcmanual_120.htm