Collection with multiple item constraints

I have a folder /details of which some page have a category.

A collection defined as follows...

items:
  - '@page.children': /details
  - '@taxonomy.category': [MyCategory]

...will contain all child pages of folder /details plus all pages with category 'MyCategory'. Since the both items are in folder /details I get all pages in /details

How do I create a collection that only contains pages from folder /details AND also have category 'MyCategory';

Perfect, it worked out of the box!
Frontmatter of the module:

pages:
    items: '@page.children': /details
taxonomies:
    items: '@taxonomy.category': [Service]

and in pages.html.wtig:

{% set pages = page.collection('pages') %}
{% set taxonomies = page.collection('taxonomies') %}
{% set collection = pages.intersect(taxonomies) %}

🙂

I just made a second module on the same modular page with

pages:
    items: '@page.children': /details
taxonomies:
    items: '@taxonomy.category': [Support]

This led to the following:

• The previous layout (two columns of mini-pages) has changed to a one-column layout.
• The second collection is not displayed at all.

What should I do in order to have multiple "page collection modules" on my modular page?

@Vince42, A few remarks...

1. The frontmatter snippet you're showing contains incorrect Yaml. If you truly use that in your page's frontmatter, an error should be thrown.

   The correct frontmatter should be:

   YAMLCopy

   pages:
    items: 
      '@page.children': /details
   

   You might want to update your posts accordingly.

2. It is a bad habit to override existing Twig variables. This may lead to hard to trace issues. pages is such a variable which is provided by Grav. See Theme Variables
3. The previous layout (two columns of mini-pages) has changed to a one-column layout.
   You are implying that a new module is changing the layout of another module. I don't see how a module could interfere with the layout of another module, unless the HTML generated by the Twig template is incorrect.
4. The second collection is not displayed at all.
   How do you know it is 'not displayed' versus 'empty'? Did you dump the collection to the debugger?
5. What should I do in order to have multiple “page collection modules” on my modular page?
   I don't know... You might want to check your Twig and check if your collection isn't empty.