Duel fuel is simply the fact your getting both your gas and electricity from the same supplier. Not all energy suppliers offer both gas and electricity, some offer just the one or duel fuel packages but not gas for instance.
By using a duel fuel tariff, in a high percentage of cases your going to find the cheapest rates. However this isn't always the case so its worth double checking what the individual service tariffs are like just in case.
Its not just the potential saving in finance terms, with a duel fuel tariff, because its just the one supplier its a little easier to manage.