If the new applicant puts your handle in the multi field, that should be sufficient for the person accepting the account to enter you along with the new applicant as a multi.
Assuming everything goes to plan, of course.
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"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
Yes, that's all - but be sure to check your Account Settings after the other account is approved since it will display you as registered multis if you were properly added. If something went wrong you won't see anyone listed as your registered multi, and at that point (if you are certain your multi's account was approved rather than declined) you should e-mail multi-accounts with the correction.