I find it better to have 2 roost bars. Sometimes they have 'issues' with each other and giving one a separate bar to get on can be helpful to prevent quarrels. Also there's the late comer to the roosting party who tries to hop up in the dark, and ends up knocking 2 others off the roost in the quickly setting dark. Drama WILL ensue!! If it's warm out, they like a little room between each other, but when it's cold out I find they like to get in closer quarters to each other. A second bar wont hurt and can absolutely help in some situations. If they want to be close, they'll get close, if they want some distance for whatever reason, it's available.
Aaron