Part FCL Appendix 6 A.9
The holder of a single-engine IR(A) who also holds a multi-engine class or type rating wishing to obtain a multi-engine IR(A) for the first time shall complete a course at an ATO comprising at least 5 hours instruction in instrument flying in multi-engine aeroplanes, of which 3 hours may be in an FFS or FNPT II.
Holder = Rating valid so you'll need to ensure both are current in advance of the ME-IR skill test.