It only takes one manager to approve a merge, but if one manager has rejected a proposed merge, that will be noted.
Let's say profile X has two managers, Manager A and Manager B.
WikiTreer C proposes that X be merged into Y.
A and B will both get the e-mail message informing them that C has proposed the merge.
A arrives first and rejects the merge.
B arrives second and sees that the two profiles are set as Rejected Matches.
B can choose to remove the Rejected Match and initiate a new merge. The original permission from C should still be stored, so B should be able to complete the merge.