Listing all entries in one section related to entries written by currentUser in another section
I think this is what you mean:

{% set sectionBEntries = craft.entries.section('B').authorId(currentUser.id) %}

{# 'relatedTo' works both ways, so we can use sectionBEntries 
    as input for relatedTo to get all section A entries that have 
    a relation to one of the section B entries by currentUser #}
{% set sectionAEntries = craft.entries.section('A').relatedTo(sectionBEntries) %}

{% for entry in sectionAEntries %}
    {{ entry.title }}
{% endfor %}