I've seen this phrase often and I always get confused by how many kids are implied in it without any other context. Is it 4 kids or 2? Because twins are a set of two, so by saying "the two twins", at least to me, it sounds like you're saying two sets of twins. But I've seen it used for just one set of twins. So, can someone please tell me which it is?
Cambridge dictionary defines 'twin' as "either of two children born to the same mother on the same occasion". Therefore, 'twin' means the individual, not the pair.
He is a twin.
They are the twins.
If you say :
There are two twins.
You mean there are two children who are twins. "Two twins" doesn't refer to four children. But we should keep in mind that the "two twins" may be or may not be twins of each other. It means they may be born to the same mother or to the different mothers.
If you want to mean 'four children', you should say :
There are the four twins.
There are two pairs of twins.
There are two sets of twins.
A twin is one of two.
Hence the two twins refers to two people.
A set of twins refers to a pair of two people. Each in the pair is a twin.
My twin is the person I am twinned with. In other words, the other person in the pair of twins.
Twin therefore carries the notion of having a counterpart, a twin being one of two.