adeuri durin je chinguneun jumalmada ttalkkaji derigo gongwone gayo.

Think of it as:

• 아들이 둘인 = who has two sons
• 제 친구는 = my friend
  • topic marker

So the whole chunk means my friend who has two sons.

A key Korean pattern here is that a clause comes before the noun it describes.
So instead of saying my friend who has two sons, Korean literally says something like sons being two my friend.

-인 comes from 이다.

The underlying idea is:

• 아들이 둘이다 = (someone) has two sons / there are two sons

When this kind of 이다 clause modifies a noun, 이다 changes to 인:

• 아들이 둘이다 → 아들이 둘인 친구
• a friend who has two sons

So 둘인 is basically two-being attached to a noun.

Also, 둘 is correct here, not 두, because this is not the direct prenoun form before a noun like 두 아들 or 두 명. It comes from the predicate 둘이다.