Roland, thanks for the comment, looks like I missed an edit.

The name of the endpoint on the mirror should be 'MirrorEndpoint'. So yes, the two connect statements should read:

create login [domain\svc-princ-sqlsvr] from windows;

go

grant connect on endpoint::MirrorEndpoint to [domain\svc-princ-sqlsvr]

go

create login [domain\svc-witn-sqlsvr] from windows;

go

grant connect on endpoint::MirrorEndpoint to [domain\svc-witn-sqlsvr]

go

The article has been edited to reflect the above changes.