C.,
Here it goes.....I have two sets of twins. The first set were COMPLETELY different like what you are saying. I ALWAYS thought there was something wrong with one of them. Comparing them is completely normal even though we are not suppose to do that! :) One seemed to talk exquisitely, the other you could barely understand. When they got to school age I separated them in kindegarten (some people find that horrifying....SORRY!)It was the best thing I could ever do, I can not explain that enough for their case. As they got older I was so hard on the one that seemed "challenged" because why wasn't she progressing like her sister, but then the "light bulb" went off in my head at a parent teacher conference-IDIOT ME, did not know one was very advanced while the other was just "normal". I had to go back and apologize, retrain and adjust my thinking.
However, the good news was the one that seemed to be deficient, flourished when separated from her sister. Her sister couldn't be there to talk for her anymore or to be dependent on her for anything. This is just the experience that I went through and I hope it is the same for you. With everything you have stated I feel you need to give it a little more time and if you are really concerned contact your pediatrician.
Were they born early?
Did they have any issues at birth?
What were their birth weights?
There are so many factors and you really did not provide much insight in that area, so it is hard to make an educated judgement call. My other set of twins we are still observing to see what happens there too! This time I am more prepared... :)
Bless you,
M.