In all the countries I've flown in it's legal, provided the approach is not a 2 NDB approach - if it says "2 NDB" at the top of the Jepp plate then you need 2 separate ADFs in the aeroplane.
In my youth I did plenty of NDBs into dark African grass airfields at night with only 1 ADF on board, never seemed to be a problem then. But I was in regular practice in those days - don't think I'd have the skills and confidence now. The joys of shiny jets with expensive equipment!
