Which twin buggy for newborns can fit on London buses and the tube?

I am having twins, live in London and don't have a car. So i'll need to take them on the bus and on London Underground - negotiating steps and escalators. Help! Tandem or side-by-side? Which brand and model? Advantages and disadvantages? Buggy/pushchair/pram must be usable for 2 newborns which limits the choices - ie Jane Twin or Nipper 360 or mountain buggy etc? Tips from those with personal experience appreciated.