Multiple repositories and Heroku

I have two git repositories (frontend and backend). Say, i need to push frontend to heroku and don't touch backend. Simple static files update, right? Should be easy. But it's not. I have to manually put my static files from frontend into backend repository and push backend to heroku. Then wait for build, env install etc. All this just to upload some files. Is there another way? Maybe some manipulations with git submodule or something like that?