Cannot seem to get code from a prerequisite repository/reference path to not show up in MP

I cannot seem to get code from a prerequisite repository/reference path to not show up in MP

I'm trying to merge 2 branches intomaster.

My origin master branch is at lp:~weebl-maintainers/weebl/+git/weebl/+ref/master (private repo)

The branches meanwhile live on another repo at lp:~autonomouse/weebl/+git/weebl/
so I've got lp:~autonomouse/weebl/+git/weebl/+ref/branch/one
and lp:~autonomouse/weebl/+git/weebl/+ref/branch/two

As branch/two is based on top of branch/one, I want to make a merge proposal that shows a diff that ignores any changes already included in branch/one. For bzr, you can do this with prerequisite branches, so I assumed you could do the same with the prerequisite branches on git.

So I make the MP for the branch/one as usual, and then when I come to make the MP for branch/two, I click on the extra options bit and add ~autonomouse/weebl/+git/weebl to the prerequisite repository and branch/one to the prerequisite reference path.

However, when I do this, the changes from branch/one are still included in the MP for branch/two.

Can you tell me where I'm going wrong?

Thanks