Béatrice Marianne Ewalds-Kvist yes usually a paper should be reviewed by two referees, but It has happened to me that one of my paper has been reviewed by three reviewers, two of them have suggested acceptance of the paper after first revision, where as I am still doing a 3rd revision to the paper based on the comment of 3rd reviewer. This is actually creating problems, as the shape of a paper drastically changes with this kind of procedures. Is there any certain global policy in this regard irrespective of journals.
Most of the good (reputed) journals have their specific policy towards acceptance of a research paper. During the review, each referee gives scoring in different sections of paper. The cumulative strength (read as SCORE) decides the final decision.
Usually, if two referee accepts, the paper is accepted. But if the editor employs/expects a third referee to comment upon, then the last comments become vital for the benefit of the paper as well as for the final decision before acceptance.
'it depends upon the Editor whether (s)he wants the manuscript to get further reviewed or may decide in favour of author if 2 of 3 referees has given positive response.