Twenty eleven post divider lines

Have you made alterations to the twenty eleven theme? If so, you should be aware that you should only alter standard themes like twentyeleven via a child theme. If you don’t do that, updates to WordPress and/or updates to the theme can overwrite your changes. You also won’t be able to ask for some types of support if you can’t return to an unchanged standard theme.

I don’t know what you mean by a dividing line between posts, because there is only one post on the page. Did you mean to say, “Between comments”?

The rule you need to change to modify the borders between post is this one:

.hentry, .no-results {
  border-bottom: 1px solid #DDDDDD;
  ...
}

Bearing in mind that you really should do this in a Child Theme, add a rule like the following to the end of the (child theme’s) style sheet:

.hentry, .no-results {
  border-bottom: /* whatever border you want here */;
}

HTH

PAE