I would suggest you read EASA Part-FCL - specifically FCL.720.H(c):
"(c) Single-pilot multi-engine helicopters. An applicant for the issue of a first type rating for a single-pilot multi-engine helicopter shall:
(1) before starting flight training:
(i) have passed the ATPL(H) theoretical knowledge examinations; or
(ii) hold a certificate of completion of a pre-entry course conducted by an ATO. The course shall cover the following subjects of the ATPL(H) theoretical knowledge course:
- Aircraft General Knowledge: airframe/systems/power plant, and instrument/electronics,
- Flight Performance and Planning: mass and balance, performance;
(2) in the case of applicants who have not completed an ATP(H)/IR, ATP(H), or CPL(H)/IR integrated training course, have completed at least 70 hours as PIC on helicopters."