I am the mother of 8 year old Fraternal twin boys. The boys are extremely close.  They are in different classes, and when I fetch them from school, I do not even bother to talk to them for the first five minutes, as they are so busy "re-connecting."  I also have an older boy, and they are all very close, but there is a definite bond between the twins.

When they were just learning to talk, Twin A (Christopher) was desperately trying to tell me something while traveling in the car, repeating the same (indistinct) word over and over. I kept trying different words (do you mean ...), but could not work out what he was saying.  

Twin B (Andrew) sat very quietly until we drove past my older son's school when he pointed to the school and very loudly (as if I was stupid) repeated the same indistinct word.  When I asked if that word meant "Michael" (my older son's name), they both smiled broadly and said yes! 

Andrew obviously knew all along what Christopher meant, but just bided his time until he could "explain" it to me!  Although they do not have a "twin language" they clearly understand each other.  Their speech has improved 100-fold, but still attend speech therapy.