Let's say I have two repos, **repo A** and **repo B**, which contains a folder with code similar to the code in repo A (doesn't really matter how this actually has happened, but OK, let's assume I've just copied contents from A). 

Now I want following:

* To work just as I got used to in repo A
* To set and additional remote repo pointing to repo B, but pushing and pulling only this subfolder.
* To push, when it needed to this repo B.

So, basically, I'm looking for the cheapest way to have something like reversed sparsed check out.