Hello,
I presume you have enabled SSO for your authentication chain (specifically passthru). In that case, this use case is essentially not supported from a technical point. Yes, you can login as a different user in the web application, but SharePoint in a SSO scenario will always use the OS credentials to authenticate. There is no configuration apart from deactivating SSO that would support acces to SharePoint as different user than the OS-logged-in-user.
Regards
Axel