Formal:
* Inseparable: This is a classic and widely understood term.
* Indivisible: This emphasizes their strong bond and how they are essentially one unit.
* Conjoined: This word implies a physical connection, but it can also be used metaphorically.
* United: This emphasizes the sense of purpose and shared goal that keeps them together.
Informal:
* Two peas in a pod: This is a common idiom that suggests they are very similar and always together.
* Attached at the hip: This implies a strong and possibly excessive closeness.
* Like two halves of a whole: This emphasizes their complementary nature and how they fit perfectly together.
* Always together, never apart: This is a simple and straightforward way to describe their constant companionship.
More specific:
* Shadow: This describes someone who is always by another's side, following them closely.
* Symbiotic: This describes a relationship where both individuals benefit from being together.
* Interdependent: This emphasizes their reliance on each other.
The best word or phrase to use will depend on the specific situation and the tone you want to convey.