How to setup two PyPI indices

I have a local GitLab installation that comes with a local PyPI server to store company internal Python packages.

How can I configure my PyPI to search packages in both index servers?

I read about .pypirc / pip/pip.ini and found various settings but no solution so far.

1. Most solutions permanently switch all searches to the other index server. But I want to be able to install and update packages from pypi.org as normal while some packages come from the local index.
2. setting multiple index servers with credentials seams to be limited to distutils (used e.g. by twine) only, but is not used by pip
3. There is confusion if to configure index servers in [global] or [install]. I assume the latter one is a rule subset for pip install. (The documentation is here unclear.)
4. While twine can reference a repository entry in the config file like -r gitlab refers to a [gitlab] section, such a named reference can't be used by pip...

So what I want to achieve:

• pip should be able to install and update regular packages from pypi.org like colorama
• pip should be able to install and update packages from gitlab.company.com
  • authentication with username (__token__) and password (7a3b62342c784d87) must work

Experiment so far:

[global]


[install]
find-links =
    https://pypi.org
    https://gitlab.company.de/api/v4/projects/2142423/packages/pypi
trusted-host =
    https://pypi.org
    https://gitlab.company.de/api/v4/projects/2142423/packages/pypi

[distutils]
index-servers =
    gitlab

[gitlab]
repository = https://gitlab.company.de/api/v4/projects/2142423/packages/pypi
username = __token__
password = geheim

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