If you can also recommend any books or professional literature on the psychological and social development of twin boys it would be greatly appreciated.
I have several additional suggestions that might help you encourage independence in your sons as they enter the first grade. For one, seriously consider placing them in separate classes. This would allow them to form independent social ties at school.
To further reinforce this process, encourage each boy to invite a classmate over for some playtime during a weekend. The more dependent child may need more of your support initially as he strives to make playmates and to develop his self-confidence.
Finally, as options become available for afterschool activities, let each boy develop and pursue his interests independently of the other. While this may mean enduring the logistics nightmare of driving to two different activities, car pooling can relieve that strain. Also, because choosing afterschool activities will require your sons to take the initiative, this is an especially fruitful way to nurture their independence and self-confidence.