gitRepoMergeInstructions.txt

Should they both be visible in one working tree as directories?
Should these be independent branches?

For instance, you can fetch one into another:

    $ cd project1
    $ git config remote.project2.url /path/to/project2
    $ git config remote.project2.fetch 'refs/heads/*:refs/project2/*'
    $ git fetch project

That will give you two (or more) branches, containing history of the
project1 and project2. They are still completely independent, just use
the same object store.

You can merge them, for example:

    $ cd project1
    $ git merge project2/master

Assuming that there is no filename collisions you'll get a repo with
two merged histories (and two starting points). In case you get
conflicts you can either resolve them by editing or just move the
problematic project in subdirectory:

    $ git merge -s ours --no-commit project2/master
    Automatic merge went well; stopped before committing as requested

here will be no conflicts. Merge strategy "ours" (-s ours) does not
take anything from the branch to be merged. The coolest strategy ever.
"--no-commit" stops the operation just before committing.

    $ git read-tree --prefix=project2/ project2/master
    $ git checkout-index -a
    $ git commit

That's it. The histories are merged and the files of project2 are
placed in the directory "project2". It is a wee bit harder to browse
the history of the files: you have to give both new and "old" name of
the project2's files, as if you renamed them (that's what read-tree
with --prefix did).