To commence training for a first multi-pilot type rating, the candidate must hold a valid multi-engine instrument rating. JAR-FCL 1 states that an instrument rating is valid for 12 months and so the IR must have been issued, re-validated or renewed in the 12 months preceding the first day of the MPA type rating course.
The Licensing Skill Test for the MPA type rating includes a validation of the instrument rating only on that type. The MEIR for single-pilot aeroplanes is not affected and, if required, must be re-validated on a SPA separately from the type specific MPA IR.
Similarly, an instrument rating valid on, say, a B737, is not valid on any other aeroplane type and, if multiple type ratings are held, the IR for each type must be revalidated separately.
So -
An IR validated on a multi-engine, single pilot aeroplane is valid for 12 months on any single pilot aeroplane.
An IR validated on a single-engine, single pilot aeroplane is valid for 12 months on any single-engine, single-pilot aeroplane.
An IR validated on a multi-pilot aeroplane is valid for 12 months only on the aeroplane type in which it was re-validated