Fellow Aviator, if you are able to obtain an FAA multi engine instructor and possibly instrument instructor rating you will be able to log all of your flight time during shared time building; even under JAA regs. One pilot logs their flight time as dual given, the other as dual received. Also, it is often possible to persuade the other pilot to allow you to do all of the flying. Often the people that do such hour building want the hours, not the stick time. They just want 100-200 multi to qualify for a regional airline. For a small price some US hour builders may well allow you to do all the flying while they log the time as "safety pilot." It's quite possible to build JAA loggable multi time for about 40 quid an hour at schools such as Skymates and Ariben Aviator. You just have to be aware of how to go about it.