I've testet in my local environment with `spl_object_hash()`. Looks like it doesn't make any difference. The `$doctrine->getManager()` will return the default entity manager which is equal to the `doctrine.orm.entity_manager` service.
```
0000000047bb1fb1000000004c315f24 = $this->get("doctrine")->getManager()
0000000047bb1fb1000000004c315f24 = $this->get("doctrine")->getManager("default")
0000000047bb1fb1000000004c315f24 = $this->get("doctrine.orm.entity_manager")
0000000047bb1fb1000000004c315f24 = $this->get("doctrine.orm.default_entity_manager")
0000000047bb1c23000000004c315f24 = $this->get("doctrine")->getManager("alternative")
0000000047bb1c23000000004c315f24 = $this->get("doctrine.orm.alternative_entity_manager")
```
So it doesn't solve the problem of having multiple entity managers.

I did a little research, other bundles like FOSUserBundle solve this problem by adding a configuration option and creating a new service for the entity manager, which is then used instead of the original entity manager service. For reference:
https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Resources/doc/doctrine.md
https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Resources/config/orm.xml#L16

So that feature is something I have to implement first. There are other fragments in my code that have to be refactored, since they're also affected by that problem.