Look at this paragraph (EASA PART-FCL) :
FCL.630.H IR(H) – Extension of privileges from single-engine to multi-engine helicopters
Holders of an IR(H) valid for single-engine helicopters wishing to extend for the first time the IR(H) to multi-engine helicopters shall complete:
(a) a training course at an ATO comprising at least 5 hours dual instrument
instruction time, of which 3 hours may be in an FFS or FTD 2/3 or FNPT II/III; and
(b) section 5 of the skill test in accordance with Appendix 9 to [Part-FCL] on multi engine helicopters.
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Same thing is written in the last version of JAR/FCL 2, amend. 6
JR