Roost boards parallel? OK?
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No, not a problem at all. They can be the same height. Just make sure your chickens are able to easily jump up and down from them--8 inches sounds fine.
Ok that's great. A ladder is especially helpful if your coop houses heavy breeds, like barred rocks or buff orpingtons.
I think so. You would want to have enough distance that they are not pooping on the other board, but close enough so that they can jump across.