Rah, a Multi engine rating is a "significant change" in your experience and skill level. Therefore a Visa is required including the TSA approval and background check.
Shun, as long as you do not intend to use those hours to apply for a certificate or rating in the USA you will be ok for the occasional "lesson" which sounds more like sightseeing flights anyway.
The hours won't count in Europe anyway.
Just don't make it more then a couple of flights as that would change the intention.