Can A Shortcode Get HTML/Text From Content And Return It Twice With Added HTML?

Okay, I think I’ve figured out what you’re actually asking. You’re asking what the variable for the bit between the shortcodes. If that is all you are asking, then the answer is the parameter variable $content. You are already processing it in your code.

Also, in your example, you seem to have a third shortcode, that you aren’t processing. Is that deliberate? It doesn’t look like you really want to produce final html with that in, so this solution assumes you don’t want that.

I’m going to assume that what you want is a simple to use shortcode format such as:

[slide=ImageTitle][slideimg]http://some.domain/slide.jpg[/slideimg][slidecap]This is the caption.[/slidecap][/slide]

Which you can then process simply by adjusting your functions like so:

function shortcode_slide($atts, $content = null) {
    if(empty($atts)) {
        $img_title="";
    } else {
        // [slide=Title]...[/slide]
        // [slide="Multi word title"]...[/slide]
        $atts = $this->attributefix( $atts );
        $img_title="<h2>".trim(array_shift($atts),'="').'</h2>'; //Remove quotes and equals.
    }

    $content = wpautop(trim($content));
    return '<div class="singe_slide">'.$img_title. do_shortcode($content) . '</div>';
}
add_shortcode('slide', 'shortcode_slide');

function shortcode_slidecap($atts, $content = null) {
    $content = wpautop(trim($content));
    return '<div class="slide_caption"><div class="captExt">' . do_shortcode($content) . '</div></div>';
}
add_shortcode('slidecap', 'shortcode_slidecap');

function shortcode_slideimg($atts, $content = null) {
    $content = wpautop(trim($content));
    return '<img src="'.$content.'" />';
}
add_shortcode('slideimg', 'shortcode_slideimg');

This should produce working html.