the age difference between my boys is minutes
They are from the same liter, never seperated since birth. They get along so well and never leave each other's side pretty much. When I walk them they walk side by side, they sleep side by side, they know the same commands, the same tricks, its too cute. If you pick up one, you have to pick up the other. They usually won't lay on seperate people's laps. They need to lay together on one lap. Its adorable.
Ok I know this won't help you since you do not have access to a sibling for your baby, but I think having a 5 month old pup is a great time to add another one to the family.