The Domain Path module allows the creation of separate path aliases per domain for entities created using the Domain Access module. For example, it looks like: https://d.pr/free/i/O4Vr9z on node edit form. The path aliases per domain are implemented as separate "domain_path" content entities. It is possible to create, edit or delete path alias per domain manually here - "admin/config/domain_path" (https://d.pr/free/i/8KbeOU).
The module is ready for testing, but requires Domain Access to work correctly. For background, see issue here - https://www.drupal.org/node/2878050.