General approach is this.
You have one template where you only show the body. You use a conditional to check whether the is also extended-text. If yes, show a link to a single-entry-page.
site/index:
{exp:weblog:entries weblog="{my_weblog}" orderby="date" sort="desc" disable="member_data|trackbacks"}
{body}
{if extended}
<a href="http://{path=site/comments}">read more</a>
{/if}
{/exp:weblog:entries}
On the single-entry-page you show {body} as well as {extendend}.
site/comments:
{exp:weblog:entries weblog="{my_weblog}" disable="member_data|trackbacks"}
{body}
{extended}
{/exp:weblog:entries}