Under the FAA rules there is no helicopter multi engine rating, so you will not find any schools here in the USA offering it. What are you trying to accomplish? Do you want to use your new skills back home in Europe? if so you will want to look for a school that offers both and there are a few here. If not, there are many good schools here n the USA. You will need to attend a school that is SEVIS approved which will limit your choices a bit.
Thomlinson is a good school as is HeliOps in the Denver Colorado area. Long Horn is Denton, TX is also a good school. I have attached the links for 3 helicopter flight school search engines.
Good luck
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