Jerezflyer,
The Multi-engine IR and M/E rating are two seperate courses and cannot be combined. If you do a M/E IR course not all of the course has to be completed on the twin, most schools would do a significant amount on a simulator (fnpt I or II) or even some in a single engine aircraft. The M/E IR allows your to exercise it's priviledges in a single as well.
If you complete a S/E IR you will have to do a minimum of 5 hours M/E IR training and another full IR test to obtain a M/E IR (in addition to a M/E rating).
(The above applies to JAR).