git merge recursive theirs, how does it work?

You must use this form to pass merge strategy options:

git merge -s recursive -Xtheirs # short options
git merge --strategy recursive --strategy-option theirs # long options

Also make sure your version supports -Xtheirs, that's a quite recent feature(?)

I think the reason it's failing is that you are specifying "recursive theirs" as the strategy. "recursive" is a strategy, and when you put a space after it, "theirs" is interpreted as something git needs to merge your working copy with (eg. another branch or refspec).

I think you will not be able to specify a strategy exactly like what you want. There is a strategy called "ours" which is the opposite of what you are wanting.

The usual pattern used in this situation is to either merge or rebase to the "B" repository. From within the "A" repository's working copy, you would do a rebase, if possible (it might not be possible, if you are already sharing the git repo with other developers). A rebase would essentially roll the A repository back to a common commit within both repositories, apply "B" commits, then "A" commits on top. You'd resolve any merge conflicts along the way.

Once you go thru the pain of either merging or rebasing to the "B" repository, the future merges will be less painful.