UNHCR has no objection to safe third country agreements or agreements designed to ensure there is responsibility sharing and regional approaches so long as certain safeguards are built into those agreements and minimum criteria are present, including things like ensuring there is access to safe and efficient asylum systems in either country that is a signatory to the agreement, and that neither country will return people to situations of danger. Those are just two of the criteria that would have to be present.
UNHCR has no objection to such agreements, which seek to improve protection and asylum, being concluded by two independent sovereign states.