You can do time building in a twin and both of you log the time under the FAA system as long as one of you is under the hood. Under the FAA system, both of you can log the time, one logs the time under simulated instruments and the other pilot logs it as safety pilot.
As far as I know two pilots cannot log the flight time at the same time under CAA/JAA syste unless the plane is certified for Multi Crew.
I hope this helps. Although it is completely legal in one country, it could back fire in the other, so be cautious.