When twins develop at different rates

I took my twins to the pediatrician for their 18-month checkup last week, and I had to ask the doctor about the younger twin, Hank, and his lack of words. He says between three and five words now, and only two of them are pronounced correctly. He says "bye bye" but he pronounces it "b-baaaooooouuuu". He definitely says "hi" and "uh-oh" correctly. But anything else he says is pronounced in a really odd way.

He's also very quiet and he tends to focus on some things that seem really simple and that he should be able to do while doing something else. Like, he could sit at the coffee table and roll his tractor back and forth for half an hour without moving or paying attention to anything else. That made me worry he was autistic. Among other things that make me worry, including the fact that he has a cousin who has autism, so it's just something that I worry about.

Hank is on the right. Harry and Mitch constantly engage each other in conversation, but Hank doesn't talk to other people very often. He doesn't use words like his brother does. He grunts instead and yells.

I had never really thought about the fact that I might be worrying too much. The pediatrician brought up the fact that the symptoms I was worried about might seem worse than they really are because I'm seeing the other twin  develop not only at a normal rate but at a faster than normal rate. She told me Harry's pretty far ahead of the curve. He's more coordinated than most children his age, has a stronger grasp of language, communicates better and says far more words. Because he is developing faster than normal and Hank is developing slightly more slowly than normal, it seems like Hank is further behind than he is in reality.

I'd never thought about that, so that brought it into a new light and kind of makes me feel better about Hank.

After I got to feeling okay about the news that he was behind, I started thinking about how he would feel later on if it ended up being something lasting. He'll always have his twin to compare himself to. And vice versa if Hank ends up excelling at something Harry stinks at. That would be hard. I wonder how they'll handle it.

Anyway, the plan for now is to have an early childhood interventionist come in to evaluate Hank. Because we have to use a program that helps pay for the intervention (such a funny word for developmental evaluation, but that's what they call it), we have to do it their way. They evaluate every stage of his development, even though it's mostly his speech that is affected. After they do the evaluation, we'll discover if it's something we need to be worried about, or if it's related to his mental development or his hearing.

Our appointment with the interventionist is at the end of October. I'll be posting the results and researching whatever we find out. It looks like the worst-case scenario as of now is either a hearing problem that will require hearing aids or a speech problem that will require the services of a speech pathologist. Our pediatrician indicated that autism was not at all a concern.

Have any of you had trouble with your twins developing at different rates? Maybe the same problem I'm having with one twin being ahead of the curve and the other behind it? How do you handle it? If any of you have older children, has it affected them?