A GmbH can have more than one director. Generally, multiple directors only acting together would have the requisite power to act for and on behalf of the company. Insofar as the directors are agreeable to grant this power to a single director, it would be possible for this director to also act alone. In the event that a director is consequently able to act alone for and on behalf of the company, one refers to this power as “sole / single power (Einzelvertretungsbefugnis”).
In general, a director would not be able to represent a company when he is personally involved on the other side of the transaction (prohibition against transacting with oneself). Also, a director is not able to represent the company on one side and represent the other party on the other side of the transaction at the same time (prohibition against multiple representation). A director may be released from both the prohibition against transacting with oneself as well as the prohibition against multiple representation.