What you're trying to achieve won't work. You can not have two templates with identical names. The Module that loads the key the latest will always have priority.
You have to understand that templates are just a key inside a big array.
'view_manager' => array(
'template_map' => array(
'layout/layout' => 'my\layout.phtml'
)
)
So if you have two modules providing this configuration, it doesn't change the fact that both use the key layout/layout
. Therefore whatever Module loads later, wins.
TL/DR You can only overwrite templates, not extend them. In your case you have to create a separate template.