Redshift from a superuser login: become another user – in other words, allows a superuser to assume the identity of another user. I know this works for a superuser assuming a normal users identity, I would assume this works for a superuser assuming another superuser – not certain – left for you to test.
set session authorization 'regularuser';
Here’s the link to AWS Redshift documentation on it.
https://docs.aws.amazon.com/redshift/latest/dg/r_SET_SESSION_AUTHORIZATION.html