Maven with user settings.xml and project-specific settings.xml - maven

Question

I have several projects which point to different internal repositories. Users on one project might not be able to see the repository on the other project, so I need a project specific settings.xml for each project.
Create project-specific Maven settings indicates that I can do this by creating a MVN_ROOT/.mvn/maven.config with
--settings ./.mvn/settings.xml
and then putting the repository information inside MVN_ROOT/.mvn/settings.xml.
That seems to work and I can add those files to git and other users can use the repository automatically.
However, the repositories require validation, which requires that settings.xml contain something like
...
<server>
<id>repo-proj-1</id>
<username>me</username>
<password>secret</password>
</server>
<server>
<id>repo-proj-2</id>
<username>my.name</username>
<password>secure</password>
</server>
...
Obviously, I do not want to put this information into the git repository. ~/.m2/settings.xml is the obvious place to put that information. However, it appears that I can only have one user settings file, as my login information is not picked up when I try to build.
Is there a way to use both settings files?
This needs to work on Mac, Linux and Windows, so something like <xs:include>~/.m2/settings.xml</xs:include> (XML include, probably not the right syntax) won't work as ~/.m2 is not usually the right location on Windows.
We are using Maven 3, and if a specific recent version is required, we can require users to upgrade to that version.

Answer 1

What about to treat the secrets as environment variables?
<server>
<id>repo-proj-2</id>
<username>${user}</username>
<password>${pass}</password>
</server>
You can set them then via your build tool:
mvn -s ./build/settings.xml -Duser=xxx -Dpass=xxx clean install

Answer 2

There should be a central repository which contains SNAPSHOT's and an other one which contains releases and a group in a repository manager which combines them for consuming. This should be configured in a settings.xml located either in Users home directory or in the configuration of your CI server.
Deployment to repositories should only being done by a CI Server never by a developer.
In the CI server you should correctly setup the credentials to be hosted by the CI Server (Jenkins: Credentials Store) etc.
If you need separate settings.xml by project it is a design problem in your setup's...and will cause many issues.
The separation of different projects is a basic thing in a Maven repository which is called coordinate which is a combination of groupId and artifactId plus version.
In general a settings.xml should be located only in the users home directory which contains users credentials if needed (as already mentioned combined with encryption).