Two guys sit on opposite sides of a strange billboard.
The two guys are workers working on the billboard taking a break.
Guys who sit on a billboard are not necessarily workers.
There are other reasons to be on a billboard and they may not be working on the billboard. They could be working, not on a break, but be on a billboard to get a vantage point of a scene.
Sitting next to a billboard doesn't mean the two guys are workers working on it.