The flight Training section contains information related to FAR 61 & 91. Any information on this site is superseeded by any national or international legislation concerned or any aircraft manufactures specifications and/or Airworthiness Directives. For the actual flight training and testing the FAA uses the Practical Test Standard (PTS).
Applicability
Additional Aircraft ratings (other than on ATPL)
Aeronautical knowledge
Flight Proficiency
Aeronautical experience
Exceptions to night flying requirements
As per
pilot certificate
As per
pilot certificate
As per pilot certificate

To add a multi-engine rating to your pilot certificate you need not to pass an additional written test if you hold an airplane, rotorcraft, powered-lift or airship rating at that pilot certificate level. You need however an endorsment from an authorised flight instructor attesting that you've been found competent in the aeronautical knowledge areas and proficient on the aereas of operation for the aircraft category and,if applicable, class rating sought. Remember that your flight instructor needs to be proficient in the aircraft too (5 hours Pilot-in-command-time on make & model) before she/he can give you flight instruction. The multi engine rating can be either restricted to VFR or can have IFR privileges. It is possible to combine the two FAA checkrides (VFR & IFR).
The training consists of ground school and flight training upto proficiency of the applicant. You need to take the check-ride with a FAA designated pilot examiner. The groundschool, flight training and test can be done both in the USA or abroad, depending the availability of examiners. See Private Pilot and Commercial Pilot and Practical Test Standard.