Twins—identical and fraternal—have a unique sibling dynamic. While birth order can help determine personality and family relations, the same rules don't apply to children born only minutes apart. Their identical age often means they share the same space at home, are given the same rules, are in the same grade at school, and often have the same friends. Despite being individual and different people, a twin's identity is often partially wrapped up with their sibling's. For more information on sibling dynamics, see the SIBLING entry.

Below are a wide range of dynamics that may accompany this relationship. Use the ideas that suit your story and work best for your characters to bring about and/or resolve the necessary conflict.

Being best friends who always have each other's backs
Either party being able to intuit their twin's emotional state or physical sensations on a deeper level (sensing when they're in danger, feeling pain when they feel pain, etc.)
The twins sharing everything: hobbies, passions, values, friends, etc.
Being in frequent communication, even as adults who are physically separated
Twins who have a regular kind of sibling relationship; they're close but don't have a deeper connection
Twins sharing space but preferring to spend time with other people
A dominant twin paired with a subservient one
Twins who don't get along
Extreme competition between the two
Codependency

Sibling rivalry
Other people's expectations creating insecurity or frustration (expecting the twins to be alike in personality or aptitudes, etc.)
Having to share something important
Unfair comparisons—e.g., one person being referred to as the smart, athletic, or shy twin
A geographic move (for college, a job, etc.) for either party that creates separation difficulties
One twin being the obvious favorite of a parent, teacher, coach, etc.
One twin finding religion, getting into politics, or discovering a new passion that creates distance
One twin following a path that the other can't follow (engaging in unhealthy habits, pursuing a toxic relationship, participating in criminal behavior, etc.)
The loss of a twin