This is something that is not possible by configuration alone. Alfresco has no end-user feature to search "as another user". But it has code-level features to execute sections of code with a different user identity (AuthenticationUtils.runAs(RunAsWork, String)), and this would allow you to develop a custom search backend that executes the search as a different user and then filters the result again based on the actual user. Be aware though that this will result in a significant performance impact due to multiple permission checking and break functionality such as pagination (due to post-filtering), that you would need to compensate for or live with.
Regards
Axel