Do wooden beams separate NPC's houses?

Solution 1:

Wooden Beams are background furniture tiles, that can support torches (think of them as in-door trees).

According to the info about Housing, in order for a house to be defined, one needs to follow:

• The side walls (vertical blocks) can be made of blocks, doors or wood platforms.
• The floor and ceiling (horizontal blocks) can be made of blocks or wood platforms.
• The NPC needs at least one solid (non-platform) block to stand on at night. This block may not be directly adjacent to either of the walls of the house.

Wooden beams, as such, do not constitute boundary. Awkwardly enough, you can build a Wood Platform wall of sorts that, according to this, would constitute a boundary (for housing purposes) and still not limit you in horizontal movement (which is what I think the concept of Beams is interesting to you).