Overpass API can't to this for you, but you could run the following Overpass API query in QGIS via QuickOSM plugin and merge the resulting polygons according to the description below.
[out:json];
(
area["name:nl"="Antwerpen"]["admin_level"=6];
area["name:nl"="Limburg"]["admin_level"=6];
area["name:nl"="Vlaams-Brabant"]["admin_level"=6];
) ->.a;
(rel(area.a)["admin_level"=6];>;);out meta;
To merge intersecting polygons in QGIS, first select all polygons, then choose "Vector->Geoprocessing Tools->Dissolve" and dissolve by field "osm_type". Save the result in a new shape file and the result to your map. A new layer with a new multilinestring should appear like in the screenshot below.
Please see the [the following link for details.
http://gis.stackexchange.com/questions/65256/qgis-merge-intersecting-polygons-into-one-which-are-part-of-the-same-feature
details][1]. Here's how the result could look like with a bit of shapeburst styling:
![alt text][1]
text][2]
[1]: http://gis.stackexchange.com/questions/65256/qgis-merge-intersecting-polygons-into-one-which-are-part-of-the-same-feature
[2]: http://help.openstreetmap.org/upfiles/res2.png
