Save content as the second most recent post revision while preserving the most recent as matching the current content of the post.

save_as_revision.php

<?php
	/**
	 * Save history of content changes to post revision.
	 *
	 * @param integer $post_id Post ID.
	 * @param string  $content Content.
	 *
	 * @return bool
	 */
	function save_as_revision( $post_id, $content ) {
		// Make sure this is not already a revision.
		$post = get_post( $post_id );
		if ( ! $post ) {
			return false;
		}
		// Get old revision if it exists.
		$old_revision = false;
		$revisions = wp_get_post_revisions( $post->ID );
		if ( $revisions ) {
			// Find the last revision that is not an autosave.
			foreach ( $revisions as $revision ) {
				if ( false !== strpos( $revision->post_name, "{$revision->post_parent}-revision" ) ) {
					$old_revision = $revision->ID;
					break;
				}
			}
		}
		// Force WP to create a revision even though we haven't changed anything.
		add_filter( 'wp_save_post_revision_check_for_changes', '__return_false' );
		$new_revision = wp_save_post_revision( $post->ID );
		// If there was no existing revision, use $new_revision and create a new $new_revision.
		if ( ! $old_revision ) {
			$old_revision = $new_revision;
			$new_revision = wp_save_post_revision( $post->ID );
		}
		if ( $old_revision && $new_revision ) {
			return $this->update_content( $old_revision, $content );
		}
	}