Pairs in the resting room
Two nightstands, two lamps, symmetry around the bed. Tradition reads it as balance for a partnership; a designer reads it as symmetry the eye can rest on. It costs one extra lamp.
What the tradition means by it
Pairs are read as an invitation to partnership, and a single nightstand as a room arranged for one person. The advice is given to people who want a relationship and to couples where one person has less room than the other, which is often literally true of the furniture.
The design reading
Bilateral symmetry is the easiest arrangement for an eye to resolve, which is why it reads as calm. A bedroom is the one room with a natural axis already in it, so symmetry is cheap to achieve there and expensive to fake elsewhere.
Both sides reachable
The practical version: both sides of the bed have a surface, a light and a way to get out without climbing over anybody. A bed pushed into a corner makes one person the inside person every night, and that is a real, small, daily thing.
Where it is only tradition
Nothing here has been measured and nothing needs to be. It costs one lamp and it makes a room look considered. Graded tradition because that is exactly what it is.