it is, it's just not exposed well. in git create two branches stacked on top of each other, and then in the PR creation window, the first PR goes from master to branch-1 as normal. Then create a second PR for branch-1 to branch-2. it's a bit of futzing around with git and GitHub but it's there/works without another tool. Or you could just use the tool, up to you.
I have had trouble with GH displaying the correct diff.
I guess in most cases I don't really want to merge branch-2 into branch-1 either, I want to merge them both into master -- but first branch-1, and only then branch-2.
I don't know if that's the use-case the third party tool is focused on?
When I try that with github alone, which I have... it just gets really messy. Sometimes I need to switch the PR to a different random base branch and then back to the actually desired one to get github to update the diff when the base branch has changed since PR opened.
Not sure if I want it enough to introduce a third-party tool to my relatively small team(s).
I wish Github just had it built in.