Hold your horses. You want someone to build a service where you’d register, with a new separate set of credentials, so that you’d synchronize your email password between devices?
Well, I have to admit that this idea of having a password synchronize yourself another password becomes much more practical if you tweak it just a little bit and manage all your passwords with just one. In fact, Mozilla has one. It’s called Firefox Sync. I’d still recommend to use a real password manager like pass and either git or syncthing for synchronizing the underlying data.
Than asking for a password for a password? Yes. Than yesterday? Hardly.