# in source repo

git checkout -b <new branch> <source branch>

git filter-branch --subdirectory-filter <sub-directory path> -- -- all

git remote add <remote name> <remote URL>
  
git fetch <remote name>
  
git push <-U> <new remote> <new branch>
  
# or merge unrelated history and then push

git merge <new remote> --allow-unrelated-histories