<?php
$text = '
        *Heading 1* Text **SubHeading 1A** More Text **SubHeading 1B** Text**Subheading 1C**
        *Heading 2* Text **SubHeading 2A** More Text **SubHeading 2B** Even more text**Subheading 2C**
';
$result = array();
if (preg_match_all('/([*]+[^*]+[*]+)/', $text, $matches))
		$result = array_map(function ($v) {
				return str_replace('*', '  ', rtrim($v, '*'));
		}, $matches[1]);

print_r($result);