That's because there aren't any specifically laid out.
But first look at:
[ QUOTE ]
Sec. 61.3 - Requirement for certificates, ratings, and authorizations.
(e) Instrument rating. No person may act as pilot in command of a civil aircraft under IFR or in weather conditions less than the minimums prescribed for VFR flight unless that person holds:
(1) The appropriate aircraft category, class, type (if required), and instrument rating on that person's pilot certificate for any airplane, helicopter, or powered-lift being flown;
[/ QUOTE ]
then you have to look at the PTS:
[ QUOTE ]
FAA-S-8081-4C (Instrument PTS):
To obtain an instrument rating with multiengine privileges, an
applicant must demonstrate competency in a multiengine airplane not
limited to center thrust. The multiengine airplane that is used to obtain
unlimited multiengine privileges must have a V MC speed established by
the manufacturer, and produce an asymmetrical thrust configuration
with the loss of one or more engines. If an instrument flight test is
conducted in a multiengine airplane limited to center thrust, a limitation
shall be placed on the applicant’s certificate: (INSTRUMENT RATING,
AIRPLANE MULTIENGINE, LIMITED TO CENTER THRUST).
...
VII. EMERGENCY OPERATIONS ............................................... 1-17
A. LOSS OF COMMUNICATIONS....................................... 1-17
B. ONE ENGINE INOPERATIVE DURING
STRAIGHT-AND-LEVEL FLIGHT AND TURNS
(MULTIENGINE)............................................................. 1-17
C. ONE ENGINE INOPERATIVE—INSTRUMENT
APPROACH (MULTIENGINE)....................................... 1-18
D. LOSS OF GYRO ATTITUDE AND/OR HEADING
INDICATORS................................................................. 1-19
[/ QUOTE ]
The PTS outlines the items to be demonstrated for multi-engine instrument privileges. These are in addition to the single engine requirements and because the multi-engine pilot will have demonstrated everything required for single-engine instrument privileges he or she will also be given SE IA priviliges upon completion ofthe test. If the test had been completed in a SE the rating would have a limitation to SE only similar to the center-line thrust limitation.