user 6 overrides user5 author when editing

When user level 6 edits a post from user level 5 the post changes to the editors ownership. I think this is a bug where the original author’s ownership should be retained.
This does not happen when an admin is editing another users post (the correct original author is shown in the drop down box and sticks).
Also, does anyone know how I can have 2 users at the same level that can edit each others posts?
1.3a5
BH