There are two teams working on the same project and they realize a common problem that was already resolved by one side, but the status of work in each branch is in progress and to make a merge can be unsafe.

How to solve this problem?

There is a safe solution: merge a specific file from another branch. And there is the steps:

$ git checkout <another-branch> <path-to-file> [<one-more-file> ...]
$ git status
$ git commit -m "'Merge' specific file from '<another-branch>'"

That's it!

Here is a more detailed approach.