What does it mean to split a branch in two?

Probably to take a series of commits and decide "this one goes on branch A, this one on branch B", e.g. if you intermingled fixing bug A and B in the same branch, you could more easily go through and assign each commit to a new branch.

The existing workflow for that would be (there are several possible workflows, but this is what I would do):

  git checkout intermingled-branch
  git branch bugfix-A
  git branch bugfix-B
  git checkout bugfix-A
  git rebase -i
  # Edit the file, keep commits that fix bug A, drop commits that fix bug B
  git push origin bugfix-A:bugfix-A
  git checkout bugfix-B
  git rebase -i
  # Edit the file, keep commits that fix bug B, drop commits that fix bug A
  git push origin bugfix-B:bugfix-B