Given a `wp-cli.local.yml`:

```
@both:
  - @prod
  - @dev
@prod:
  ssh: rc~/webapps/production
@dev:
  ssh: v/srv/www/runcommand.dev
```

One can:

```
$ wp @both rewrite flush
Success: Rewrite rules flushed.
Success: Rewrite rules flushed.
```