Git: How to rebase one remote off another remote

Question 1

Since these are remote branches (and assuming that you are not working in isolation), it is a very bad idea to rebase one onto the other.

This is because rebasing "B" onto "A" will recreate the entire history of "B". Anybody else who will retrieve the remote "B" will now have to deal with that (usually that means they are seeing conflicting changes). You will be making live miserable for your collaborators.

To pick up the changes from "A" and incorporate them into "B", checkout your local version of "B", and merge "A" into it. Then push the updated "B" to the remote repository.

As a general rule: do not rebase branches that are already in a remote repository, unless you're the only person working with that remote.

Question 2

What are the best practices for this type of situation?

That mainly depends on whether there is a chance that other people already have seen the commits in both branches.

If one branch—say, RemoteB—is on your local machine and has never been pushed to a public repository, you can safely rebase it on top of the other branch:
```
git checkout RemoteB
git rebase RemoteA
```
Otherwise, the usual method is to merge one branch into the other:
```
git checkout RemoteB
git merge RemoteA
```