I think they can often and easily be used as synonyms. You can also use "dull" to mean not very smart or bland. For example, "dull" is also opposite to "sharp," say when describing a knife's edge. So you can use "dull," in that sense, more abstractly as well, to describe a person (not so nice) or a thing.
With regard to people who are not interesting to listen to, there's very little difference and you can use the two interchangeably.
To me, there is a slight difference. A "dull" person just isn't interesting. A "boring" person is a dull person who insists that you go on listening to him. He demands your attention. He "buttonholes" you. You can't escape from him.