So basically have repo A depend on repo B, and Pants in repo A downloads repo B and can operate on its Pants targets as-if first party sources?
There is no such functionality today. Usually that is done via publishing from repo B and consuming in repo A. Could you elaborate a little more on what you'd like to see? And could you open a ticket for it at https://github.com/pantsbuild/pants/issues/new/choose ? That way we can track ideas, get votes on them etc.