I wouldn’t worry yet, but you are wise to be observant and aware of the disparity between your boys. It is too early to tell whether this disparity heralds a significant difference in their developmental potentials. From what you write, it sounds as though the main difference is in motor (or large muscle) development. Achievements in this area are not very good indicators of developmental speed. According to your description, the “slower” twin “babbles all the time.” This can actually be a better indicator of overall potential.
It is very important to give twins individual attention. It is easy to fall into a pattern of thinking and talking about “the twins” rather than “Bob and Jim” (to make up names for your boys). Find opportunities to do things separately with each boy, if only for 15 minutes a day. When your husband is at home and you need to go shopping, take just one of them with you, remembering to take the other the next time.
In some of your individual sessions with the twin whose motor development lags behind his brother’s, put him on the floor on his tummy and place your hand against his feet as he kicks and thrusts. Put an appealing toy just out of reach. You may be surprised to find him reaching the toy. Another wonderful motor development game is to put him on the floor on his back, take his hands and slowly pull him to a sitting or standing position. Make up a name for the game and say it each time you pull him up. In my family we called it “Hum up, Queenie.” (Don’t ask me why.) Also, read books to both boys. There are many wonderful books for infants available at libraries and bookstores. Talk to your sons at every opportunity, as twins tend to lag a little behind singletons in speech development.
One more thing I should mention: I am the mother of twins, and I know what you’re going through. In my case, it was the first-born who was much slower in motor development. But he caught up and soon passed his sister. The same sort of thing could well happen with your boys.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.