Template code to split a post and print a custom field?

I’d suggest doing it this way to avoid running into problems where you split within a sentence/paragraph.

Try splitting on <p> – then put the first element (first paragraph) into <div class="content-first-half"> and the rest (if any) into <div class="content-second-half"> in your template.

It should be something like this (assuming $content has the html for the post)

$paragraphs = preg_split('/<p/i', $content);
array_shift($paragraphs); // ( remove first empty element
$first_half="<p" . array_shift($paragraphs);
$second_half="<p" . implode('<p', $paragraphs);

I chose <p to split on since presumably the content will be formatted into paragraphis with <p> tags – but could have inline styles/classes attached, so you can’t split on a <p>.

As far as getting the post’s custom field, this should be a good start.