As long as author 2 doesn't check in the changes, author 1 will have old topic b on his system (and in the version control server). You will get the exact situation as you describe.
As soon as author 2 checks in, that version is the latest version. And you will get that from RH, unless you specifically get an old version from version control. But that is cumbersome and very error prone.
Thanks, Willem. I didn't think of that as an option, because we have "Check in all files when closing a project" enabled in RH settings. So right now, every author always has the latest version of all files. But yes, if we disable that option, that might be a simple solution.
My concern is that we will run in a lot of version conflicts with that option disabled. Not only topic-related, but also related to project files like rhbag.apj, root.fpj etc. These files will no longer be automatically synced - is that really manageable? Do you know any authoring team that works like this?
In the update all files dialog, an author can uncheck the files he wants to keep checked out. That would only mean the topics in your case. So that might be a way. That is how some people work. But most of what I've encountered is having this issue with existing topics. And not checking in seems to work most of the time.
Otherwise, you can use conditional tags. If every author has a tag like Author_1_edit, you can exclude these tags in the output. When someone works on a topic, all new content or changes can be tagged.
Thanks, that helped a lot. We will evaluate the options.