Normally, this is the solution for the problem:
You can give both authors permissions to change permissions of blogs and blog posts. This way, when one of the authors creates a post, he can immediately change its permissions and remove the permissions of the other author over this blog post. Now the second author won't be able to modify the post or change its permissions. When he creates a post himself, he can do the same thing.
However, in the current release of Sitefinity, there is a bug with permissions, which will be handled soon.
I am sorry for the inconvenience.
All the best,
the Telerik team
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