Hi Rene,
When you test in mapping tool, you have to include the test message starting with root node(say MT_ABC) after the Message tags.
for example:
It should look something like
<?xml version="1.0" encoding="UTF-8"?>
<ns0:Messages xmlns:ns0="http://sap.com/xi/XI/SplitAndMerge">
<ns0:Message1>
<MT_ABC>
<your sub nodes/data here>
</MT_ABC>
</ns0:Message1>
</ns0:Messages>
However, for IDocs, I believe you need to use external definition since you want IDocs as multiple and not the root node above that.
I have not tried IDoc at target without an external definition and changing the signature to 0..unbounded but you can try by just mapping Message node to a constant and creating multiple nodes above IDoc node.
For example
Hope I was able to understand your query and it helps!
Ambrish