field-type.php

/**
 * Gets a number of posts and displays them as options
 * @param  array $query_args Optional. Overrides defaults.
 * @return array             An array of options that matches the CMB options array
 */
function cgc_get_posts_for_cmb( $query_args ) {

    $args = wp_parse_args( $query_args, array(
        'post_type' => array('post'),
        'numberposts' => -1,
    ) );

    $posts = get_posts( $args );

    $post_options = array();

    if ( $posts ) {
        foreach ( $posts as $post ) {

        	$type = $post->post_type;

           	$post_options[] = array(
               'name' => strtoupper($type).': '.$post->post_title,
               'value' => $post->ID
           	);
        }
    }

    return $post_options;
}

test.php

class cgcFlowsMeta{

	public function __construct(){
		add_filter( 'cmb2_meta_boxes', array($this,'flows_meta') );
	}

	public function flows_meta( array $meta_boxes ) {

		$meta_boxes[] = array(
			'id'	=> 'cgc-flow-setup',
			'title' => __('Flow Setup', 'cgc-flows'),
			'object_types' 	=> array('cgc_flows'),
			'fields' => array(
				array(
					'id' 			=> 'cgc_flow_tracks',
					'name' 			=> __('Flow Tracks', 'cgc-flows'),
					'type' 			=> 'group',
					'options'     => array(
						'group_title'   => __( 'Track {#}', 'cgc-flows' ), // {#} gets replaced by row number
						'add_button'    => __( 'Add Another Track', 'cgc-flows' ),
						'remove_button' => __( 'Remove Track', 'cgc-flows' ),
						'sortable'      => true
					),
					'repeatable'     => true,
					'desc'			=> __('', 'cgc-flows'),
					'fields' 		=> array(
						array(
							'id' 	=> 'track_connections',
							'name' 	=> __('Post to Connect', 'cgc-flows'),
							'type' 			=> 'select',
							'options'     => cgc_get_posts_for_cmb(array('post_type' => array( 'cgc_quiz','cgc_courses') )),
							'repeatable'     => true
						)
					)
				)
			)
		);

		return $meta_boxes;

	}

}
new cgcFlowsMeta;