so I was able to migrate the customers and orders into the magento 2 again, without needing to use the delta migration step, nor empty the existing magento 2 orders/customers. I think it just overwrites the existing entries.
this is my config.xml data step looks like
<steps mode="data">
<step title="Data Integrity Step">
<integrity>Migration\Step\DataIntegrity\Integrity</integrity>
</step>
<!-- <step title="EAV Step">
<integrity>Migration\Step\Eav\Integrity</integrity>
<data>Migration\Step\Eav\Data</data>
<volume>Migration\Step\Eav\Volume</volume>
</step> -->
<step title="Customer Attributes Step">
<integrity>Migration\Step\Customer\Integrity</integrity>
<data>Migration\Step\Customer\Data</data>
<volume>Migration\Step\Customer\Volume</volume>
</step>
<step title="Map Step">
<integrity>Migration\Step\Map\Integrity</integrity>
<data>Migration\Step\Map\Data</data>
<volume>Migration\Step\Map\Volume</volume>
</step>
<!-- <step title="Url Rewrite Step">
<integrity>Migration\Step\UrlRewrite\Version191to2000</integrity>
<data>Migration\Step\UrlRewrite\Version191to2000</data>
<volume>Migration\Step\UrlRewrite\Version191to2000</volume>
</step>
<step title="Log Step">
<integrity>Migration\Step\Log\Integrity</integrity>
<data>Migration\Step\Log\Data</data>
<volume>Migration\Step\Log\Volume</volume>
</step>
<step title="Ratings Step">
<integrity>Migration\Step\Ratings\Integrity</integrity>
<data>Migration\Step\Ratings\Data</data>
<volume>Migration\Step\Ratings\Volume</volume>
</step>
<step title="ConfigurablePrices step">
<integrity>Migration\Step\ConfigurablePrices\Integrity</integrity>
<data>Migration\Step\ConfigurablePrices\Data</data>
<volume>Migration\Step\ConfigurablePrices\Volume</volume>
</step> -->
<step title="OrderGrids Step">
<integrity>Migration\Step\OrderGrids\Integrity</integrity>
<data>Migration\Step\OrderGrids\Data</data>
<volume>Migration\Step\OrderGrids\Volume</volume>
</step>
<!-- <step title="Tier Price Step">
<integrity>Migration\Step\TierPrice\Integrity</integrity>
<data>Migration\Step\TierPrice\Data</data>
<volume>Migration\Step\TierPrice\Volume</volume>
</step> -->
<step title="SalesIncrement Step">
<integrity>Migration\Step\SalesIncrement\Integrity</integrity>
<data>Migration\Step\SalesIncrement\Data</data>
<volume>Migration\Step\SalesIncrement\Volume</volume>
</step>
<!--<step title="PostProcessing Step">
<data>Migration\Step\PostProcessing\Data</data>
</step>-->
</steps>
this is the map.xml looks like
<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
* Copyright © 2013-2017 Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
-->
<map xmlns:xs="http://www.w3.org/2001/XMLSchema-instance" xs:noNamespaceSchemaLocation="../../map.xsd">
<source>
<document_rules>
<ignore>
<document>catalog_category*</document>
</ignore>
<ignore>
<document>catalog_compare*</document>
</ignore>
<ignore>
<document>catalog_eav_attribute</document>
</ignore>
<ignore>
<document>cataloginventory_stock*</document>
</ignore>
<ignore>
<document>catalog_product_*</document>
</ignore>
<ignore>
<document>catalog_category_*</document>
</ignore>