If someone is tolerant, in the sense that they accept, and do not wish to change, views held by others which are different from their own, are they obliged to tolerate intolerance in others?
I think the matter is a little more complicated. There is a sense of "tolerance" that means "indifference," as in "I don't see why those parents tolerate that kind of behavior from their children." This is the sense I think people have in mind when they say, plausibly, that no one should tolerate racism, sexism, or, for that matter, intolerance. But this isn't the sense in which "tolerance" is a virtue. The kind of tolerance required for a civil society is the kind that acknowledges the rights of every person to make up her own mind, to form her own opinions, and that makes illegitimate any attempt to coerce anyone to change her mind. In this sense, there's no incompatibility between tolerating a racist belief, and trying, by all legitimate means of persuasion, to change the opinion of the person who holds it. In this sense, a tolerant person should tolerate intolerance -- but she doesn't need to ignore it or endorse it, and can do her best to argue against it at every opportunity.