You only need one complex endorsement under FAA regulations,
§61.31 Type rating requirements, additional training, and
(c) Aircraft category, class, and type ratings: Limitations on the
carriage of persons, or operating for compensation or hire. Unless a
person holds a category, class, and type rating (if a class and type
rating is required) that applies to the aircraft, that person may not
act as pilot in command of an aircraft that is carrying another
person, or is operated for compensation or hire. That person also may
not act as pilot in command of that aircraft for compensation or hire.
(e) Additional training required for operating complex airplanes. (1)
Except as provided in paragraph (e)(2) of this section, no person may
act as pilot in command of a complex airplane, unless the person has—
(i) Received and logged ground and flight training from an authorized
instructor in a complex airplane, or in a flight simulator or flight
training device that is representative of a complex airplane, and has
been found proficient in the operation and systems of the airplane;
(ii) Received a one-time endorsement in the pilot's logbook from an
authorized instructor who certifies the person is proficient to
operate a complex airplane.
In your case a class rating is only required for the "multi" aspect of the DA-42 as the DA-42 has no type rating.
The FAA further more defines complex aircraft as
Complex airplane means an airplane that has a retractable landing
gear, flaps, and a controllable pitch propeller, including airplanes
equipped with an engine control system consisting of a digital
computer and associated accessories for controlling the engine and
propeller, such as a full authority digital engine control; or, in the
case of a seaplane, flaps and a controllable pitch propeller,
including seaplanes equipped with an engine control system consisting
of a digital computer and associated accessories for controlling the
engine and propeller, such as a full authority digital engine control.
No part of this definition stipulates how many engines the aircraft has.