I have the following setup for two collections:
permalink: /blog/{slug}/
template: index
filter: tag:blog
data: tag.blog
/release-notes/:
permalink: /release-notes/{slug}/
template: index
filter: tag:release-notes
data: tag.release-notes
I would like my release-notes to be shown in both, the /blog/ page as well as the /release-notes/ page.
However, it seems that when I tag my release notes with both, blog + release-notes, it will only ever appear in one or the other.
Is there a way to change that?
/blog/:
permalink: /blog/{slug}/
template: index
filter: tag:blog+tag:lol
data: tag.blog
/release-notes/:
permalink: /release-notes/{slug}/
template: index
filter: tag:blog+tag:release-notes
data: tag.release-notes
It seems like you’re looking for channels 
Ghost Themes - Dynamic URLs & Routing.
Posts can only be assigned to one collection
Oh! Well I think that was it but now all posts in my channels send me to the 404 page?!
Am I doing something wrong here? This is my new Setup:
routes:
/: landing
/docs/: custom-post-docs
/blog/:
controller: channel
filter: tag:blog
/release-notes/:
controller: channel
filter: tag:release-notes
I am still defining taxonomies and collections below. Is that a problem?
It seems like you want the primary URL to be /blog/a, but you also want to mirror the release notes to /release-notes/a. For that case, the blog route should not be a channel.
Channels do not provide posts with a home which is probably why you are getting the 404
Right. I got it now! Thanks!
Is this not explained in the docs or did I miss that?
Collections and channels take a little bit of time to get used to :) There is this section in the docs though
Unlike collections, channels have no influence over a post’s URL or location within the site, so posts can belong to any number of channels.
Ah yeah there it is. Thanks for the help again.