Add multiple wp_editor() inputs to a single metabox. Save as single meta value.

functions.php

/**
 * Adds a box to the main column on the Post and Page edit screens.
 */
function myplugin_add_meta_box() {

	$screens = array( 'post', 'page' );

	foreach ( $screens as $screen ) {

		add_meta_box(
			'myplugin_sectionid',
			__( 'My Post Section Title', 'myplugin_textdomain' ),
			'myplugin_meta_box_callback',
			$screen
		);
	}
}
add_action( 'add_meta_boxes', 'myplugin_add_meta_box' );

/**
 * Prints the box content.
 * 
 * @param WP_Post $post The object for the current post/page.
 */
function myplugin_meta_box_callback( $post ) {

	// Add a nonce field so we can check for it later.
	wp_nonce_field( 'myplugin_save_meta_box_data', 'myplugin_meta_box_nonce' );

	/*
	 * Use get_post_meta() to retrieve an existing value
	 * from the database and use the value for the form.
	 */
	$value = get_post_meta( $post->ID, '_my_meta_value_key', true );
	?>


	<div class="my_meta_control">
 
	<p><?php _e('Text editor #1', 'myplugin_textdomain');?></p>
 
	<p>

		<?php 
		$one = isset($value['one'] ) ? $value['one'] : '';

		$settings = array(
			'textarea_rows' => '10',
			'media_buttons' => true,
			'textarea_name' => 'myplugin_new_field[one]',
		);

		wp_editor( $one,  'myplugin_new_field_one' , $settings );
		?>

		 <span><?php _e( 'Enter in some text', 'myplugin_textdomain' );?></span>
	</p>

	<p>

		<?php 
		$two = isset($value['two'] ) ? $value['two'] : '';

		$settings = array(
			'textarea_rows' => '10',
			'media_buttons' => true,
			'textarea_name' => 'myplugin_new_field[two]',
		);

		wp_editor( $two,  'myplugin_new_field_two' , $settings );
		?>

		 <span><?php _e( 'Enter in some text', 'myplugin_textdomain' );?></span>
	</p>

	<p class="meta-save"><button type="submit" class="button-primary" name="save"><?php _e('Update');?></button></p>

</div>

<?php
}

/**
 * When the post is saved, saves our custom data.
 *
 * @param int $post_id The ID of the post being saved.
 */
function myplugin_save_meta_box_data( $post_id ) {

	/*
	 * We need to verify this came from our screen and with proper authorization,
	 * because the save_post action can be triggered at other times.
	 */

	// Check if our nonce is set.
	if ( ! isset( $_POST['myplugin_meta_box_nonce'] ) ) {
		return;
	}

	// Verify that the nonce is valid.
	if ( ! wp_verify_nonce( $_POST['myplugin_meta_box_nonce'], 'myplugin_save_meta_box_data' ) ) {
		return;
	}

	// If this is an autosave, our form has not been submitted, so we don't want to do anything.
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
		return;
	}

	// Check the user's permissions.
	if ( isset( $_POST['post_type'] ) && 'page' == $_POST['post_type'] ) {

		if ( ! current_user_can( 'edit_page', $post_id ) ) {
			return;
		}

	} else {

		if ( ! current_user_can( 'edit_post', $post_id ) ) {
			return;
		}
	}

	/* OK, it's safe for us to save the data now. */
	
	// Make sure that it is set.
	if ( ! isset( $_POST['myplugin_new_field'] ) ) {
		return;
	}

	// Sanitize user input.
	$my_data = array();

	foreach( $_POST['myplugin_new_field'] as $i => $value ){
		$my_data[$i] = sanitize_post_field( 'post_content', $value, $post_id, 'db' );
	}

	// Update the meta field in the database.
	update_post_meta( $post_id, '_my_meta_value_key', $my_data );
}
add_action( 'save_post', 'myplugin_save_meta_box_data' );