Those Two are Triplets [duplicate]

Solution 1:

It may sound odd to say, but the reason that 'triplets' is plural is because two people are in the subject.

Going off of your example with 'twin', you could say:

"Those two are twins."

which is perfectly valid because you are talking about two people. The word 'twin', like the word 'triplet', is a singular word that must become plural when it is used with a plural subject.

Solution 2:

Taking your question a step further, most readers who encounter the sentence "They are triplets" will assume (in the absence of contextual information to the contrary) that the "They" refers to three people. But what's actually going on in that sentence, I think, is an extreme compression of the idea "They are members of a set of triplets"—in which case, the number of people that "They" refers to in a particular instance may be either two or three.

By the same reasoning, "Those two are triplets" is equivalent to "Those two are members of a set of triplets," a longer form that most people will have no trouble understanding at once. As sometimes happens, the compressed form here is technically correct and yet may bring the narrative to a sudden stop as readers try to work out whether and how it makes sense. Faced with that likely result, an author who doesn't find the avoidable jolt in the narrative's progress desirable would do well to use a longer form of the expression.