I'm not referring to "one of a kind" or "first of a kind" but "the two were of a kind". This turns out to be remarkably un-Googlable so I am having difficulty providing examples. In context I have always assumed it meant that two things were in a unique category unto themselves.
"Two of a kind" means that there are two parts that make up the total. "The two were of a kind" is a little different because it doesn't imply that there are only two in that "kind", but that those two are in that "kind", where kind generally refers to a unique personality trait, state of mind, or way of looking at the world.
It really means the same as it does in the context of the phrases "one of a kind" and "first of its kind." In this context, "kind" means (loosely) "a type or variety having specific quantifiable features." So for "the two" to be "of a kind" means that they have specific quantifiable features in common. The phrases "of a kind" and "of the same kind" are more or less semantically interchangeable.
It basically means that they share some common subset.
It can be used in a manner to exclaim that a group is special and out of the ordinary.
It is often used to express distaste at certian life choices in a manner that seems less overt or crass. In this manner the exact definition is left indefinate so that the speaker can claim misunderstanding.