Be/live in each other’s pockets
Definition 1 of 1
Pro Tip 1/3Close Association
Implies a very close and possibly overly dependent relationship between people. 
The twins are always together; they really live in each other's pockets.
Pro Tip 2/3Potential Negativity
May suggest lack of independence or clinginess in a relationship. 
It's not healthy that they live in each other's pockets; they need some time apart.
Pro Tip 3/3Colloquial Usage
Used informally, more often in casual conversation than in formal writing. 
They're roommates and best friends, so they're always in each other's pockets.