You will need to contact firstname.lastname@example.org to have your accounts merged.
FYI - for anyone else reading this, please do NOT create multiple accounts as it will only delay your results.
This is on the login page:
IMPORTANT: Having multiple accounts causes delays in receiving exam results. Creating multiple accounts to avoid a rescheduling fee, circumvent the (ISC)² Retake Policy or to reapply a free exam code is a violation of (ISC)² Exam Policies and may nullify your exam results and/or result in a temporary or permanent ban from (ISC)² exams.
(ISC)² Community Manager